It's rather strange to me to hear people claim there are no atari monitors...
The thing is, I had an 800XL in 1990. (I have one now too, but it's not the same one.)
To be more precise I had two of them (both 800XL), a bunch of games, and some accessories. (about 400 games, roughly)
Two 1010 program recorders, a 1020 plotter, Atari touch tablet....
And an atari branded monitor!
I have to point out that this was in the Netherlands, but I'm quite certain the monitor was atari branded.
For good measure it was a green monochrome monitor. (though obviously the 800XL has no chroma output anyway, so that makes sense. - a colour monitor for an unmodified XL machine would be pointless.)
The thing is, I used this monitor exclusively back then, to the point that up until recently I'd never actually seen a single atari game in anything other than monochrome.
Now, what this monitor actually is, I couldn't tell you retrospectively. (I no longer own it, and I was about 7 when I did. My memory for stuff like model numbers simply isn't that good. XD)
Still, unless my memory is playing some very serious tricks on me, this means at least one dedicated atari monitor exists for their 8 bit line.
(monochrome with green phosphors) - given the systems I had it with, and all the other accessories I got with those systems, it's reasonable to assume this is a monitor that dates to the same period as the XL computers, though I have no proof of this.
(it also helps that the monitor housing, as far as I remember was beige with dark brown accents - in other words, the same colour scheme as the XL machines themselves.)
Unfortunately, unless I somehow get lucky and find a reference to it somewhere else, I guess I can't particularly prove it exists.
Unfortunately, unless you find someone who has kept things for 30-40 years, it's a bit difficult to prove certain things even existed. (Especially if they're rare or obscure)
As some extra information, I'll describe the monitor, and see if it seems at all familiar to anyone. Keep in mind I have no references for it. (pictures, model numbers, etc.). This is entirely going off my memory of things nearly 28 years later. As such it may not be entirely accurate.
The display was monochrome green and the overall size was very similar to a 34 cm television we had at the same time. (suggesting a ~13 inch display, but obviously 12 and 14 are in the same bounds.)
It had audio capabilities, though I don't remember where the speaker was placed.
Most of the case was an cream colour, similar to an XL machine, with at least part of the front section and bezel being a darkish colour (in my memory it's the same shade as the brown on an XL machine)
All the controls were along the bottom of the display.
On the far right side you had either two or three knobs each about 2-3 cm across, where only the bottom half of the knob protrudes from underneath the case.
The rightmost knob was either brightness or volume (can't remember which), but also the power switch; From the off position, you rotate it a little, there's a rather strong 'click', and the monitor switches on. All further rotation controls brightness (or volume?). If it was two knobs then the other one was whatever the main knob wasn't. (eg if the main knob was volume, the other is brightness and vice versa.).
The third knob, if it was there, was contrast.
These knobs were in the darkish, brown colour.
In the centre, you had a fold down panel, just like a lot of CRT monitors and TV's had. (push the right hand side to release, then fold it down) This was, I believe beige. Right in the middle of it you had a small atari logo in rainbow colours.
If you opened the panel there were more rotating knobs, though these were long and thin.
I can't remember how many, but at a minimum it had horizontal and vertical hold controls. (and if there wasn't a third large knob, then contrast was in here too.)
As I said, my memory of things is a little dubious, but that's what I remember of it. If this sounds at all like a monitor anybody has seen, I'd be curious to know what you think it may have been...
I'm sure I'd know it if I saw it, but so far I haven't seen any pictures of atari monitors that look like what I remember.