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Many things around the '80s or earlier use ‘/’ in their abbreviations and sometimes even in their names, for example
Nowadays it's typically used to express or or (sub)division so I find it a little bit weird. Why was it used like that?
Well, at least on some of those, it's a quite natural divider, such as you would see in:
For your cited examples, they are read as:
CP/M, from Digital Research Inc (DRI), was the control program for microcomputers, though it may have originally been control program and monitor. See here or, if you want it direct from Gary Kildall, his "Computer Connections: People, Places, and Events in the Evolution of the Personal Computer Industry" manuscript (available at the Computer History Museum) states:
So, I built an operting [sic] system program using the simulator. I called it CP/M, or a Control Program for Microcomputers, mimicking the name PL/M. For me, CP/M's sole purpose was to support the PL/M language. Nothing else.
MP/M, multitasking program for microcomputers, though multi-programming monitor and control program for microcomputer systems development in DRI's original specification.
Several others were also from DRI and followed a similar nomenclature: CP/NET and MP/NET were similar to the preceding two but meant to operate over networks, hence control/multitasking program for networks.
PL/M, programming language for microcomputers, foreshadowed above in the CP/M bullet point but more directly in the same document:
PL/M stands for a "Programming Language for Microcomputers," and is still used by Intel customers today, though largely supplanted by the "C" programming language.
PL/S, programming language for systems. See references on the Wikipedia entry. Note that this was eventually replaced with PL/X, which is, at least up to several years ago when we parted ways, IBM's internal-use-only (I think) language for today's mainframe.
OS/8, the OS for the PDP-8. PS/8 I'd never heard of but it looks like (from cursory investigation) an early name for OS/8. The Wikipedia entry supports this as does the fact a later OS/12 was built for the, wait for it, ... , PDP-12 :-) An unofficial history FAQ also cites these, stating that OS/8 came from the unfortunately named (though probably intentional) "Fully Upward Compatible Keyboard Monitor", or *BLEEP* MONITOR :-)
MS/8 was also similar in that it was the monitor system for the PDP-8.
Contrast this "something something for something" approach with the (albeit fictional) MCP master control program from the original (and much better despite the much less modern graphics) Tron movie :-)
Some others, coming from IBM, have a similar format, seemingly dividing the general term from the specific instance:
The PS/2 port, which is a type of connector into which mice and keyboards are plugged into, is so named because it was first introduced on the PS/2. It is a mini-DIN connector. Previously (and this is before USB prevalence), PCs generally used a larger (DIN) connector.
Control Program/Monitor and even when reading / as for I still find it weird
Control Program/Monitor it read Control Program and Monitor
CP/M is representing Control Program for each Microcomputer then sure. But that doesn't seem likely. You've expanded your examples, but still not given any that map to "for" in the way you've then used it in your answer.
In addition to Paxdiablo's computing centred list it might be worth to add that it's more of a linguistic issue than a technical or computing one.
The Slash was quite present way before data processing, back then known as Solidus or Oblique. Solidus, Latin for Shilling, as it grew out of the long-s, a letter, lost to modern Alphabets, used separate Shilling from Pounds. The long-s looks a bit like a stroke with a hook. The hook got lost over time due sloppy writing, transforming it into a straight slash. Solidus is in fact the name used within the Unicode classification for what we commonly call a slash.
Development and use case goes roughly:
Slashes have been used as number separator long before Comma and Dot settled like today. This dates back to at least the 16th century.
The usage is originated in the fact that in handwriting spaces can be rather random in width and appearance. Thus, separation is needed/helpful, some symbol gets inserted. Either a specific, or a generic like a slash.
In fact, in many early Latin texts and many medieval handwriting spaces were optional or omitted at all. If a word separator was needed, like to switch for numerals, a middle dot was common.
Slashes are still common in some countries (e.g. USA) as date separator (YY/MM/DD).
It was and is used wherever one wants to concatenate items but still mark em as distinct. Like the PL/I example, were it serves to enable reading 'I' as one instead of the letter I.
Similar the use with other product names like System 360 abbreviated to S/360. This continues with S/360-67 as abbreviation for the System 360 Model 67. Here a hyphen is used as secondary divider
Usually these applications are about saving space, so the slash may as well serve as replacement for various words. They can stand for and, or, for or many others.
This essentially works like the title case used in English language publications. All words not capitalized are candidates to be left out at whole, or replaced by a slash. Like Control Program for Microcomputers becomes CP/M.
In addition, much like there isn't one title case, but each publication fosters their own, people tend to make up their own variation - for example it's common to write simply 360/67 instead of S/360-67.
Speaking of, even companies change mid way, as there are as well IBM publications writing like that - or turning everything upside down by creating z/OS for something that started out as OS/360 :))
Of course all of this is in addition garnished with heaps marketing related stylization, great for flashy advertisement and even better to play trademark games.
The whole thing goes not only for products, but company names as well, or who remembers that M/A/I (sometimes M|A|I, today MAI) originally stand for Management Assistance Incorporated?
Long story short: Once it was about abbreviations, today everything goes, use it whenever pleases your intention.
Now, when looking at computing in particular, then there is a clear lineage, at least for some of the usage:
IBM stylized PL/I to fit the scheme started with the together with the S/360
Which in turn may have originated in use by US military bureaucracy like Another-Dave pointed out.
Intel's PL/M is a simplified implementation of PL/I, so it's name follows the precedence set.
PL/M was written for Intel by Garry Kildall. When creating CP/M, not long after, the naming choice was obvious.
Not much sleuth skills needed to see the continuation in MP/M and all that followed in DR's timeline of OSes.
This answer doesn't really answer the question of "why". But in any case....
In technical/computing fields, the solidus appeared in US military project designations. A few examples:
AN/FSQ-7 - , vacuum tube computer, part of the SAGE air defense system.
AN/FSQ-32 - solid state computer, also SAGE.
AN/FSG-1 - anti-aircraft defence system.
The structure of such names is that "AN" means "Army and Navy", "FSQ" is "Fixed Special eQuipment", and so on. The numeric part is a specific project.
It seems to me that the solidus is essentially an arbitrary choice of punctuation symbol to separate two logically distinct parts of the name, as is the hyphen before the numeric identifier. There's no more to it than that.
/ in case of separation because I already mentioned (sub)division in the question, but it's not like that for CP/M or PS/2 or most of the names I listed above
It is being used in the same way as you describe the modern usage- to express a subdivision of a whole.
In the case of your cited examples, the division is between the Domain and the Specific instance. Domains are things like "OS", "CP", "PL", etc. and the part after the "/" names the specific instance. So, you can best understand it as:
"In the Domain of Control Programs, here is the Microcomputer specific one- CP/M".
-OR-
"In the Domain of Operating Systems (for IBM PC's), here is specific revision number 2- OS/2".
It's rather simple and logical. The answer by another-dave makes this point as well, using examples that start with "In the Domain of Army and Navy gear..."
(sub)division. It's nothing new but I'm not convinced that most of the names are in the domain/specific format
S/360seems to be a favorite pattern (general/specific) of IBM, they also hadS/34and all their other names likePS/2,OS/2,PL/I. But, of course, they seemed to brreak that withz/OSfor some reason.