On today's keyboards, the two Shift keys are somewhat elongated horizontally, but otherwise look much the same as any other key on the keyboard. However, in times past it was not altogether unusual for Shift keys to have quite a different profile: overall the key was wider than the other keys, but the left and right edges were recessed. The centre of the key jutted out from this recessed base, with a pressable surface that was about the same size, or maybe only slightly wider, than a normal alphanumeric key. Below is an example of such a keyboard (namely, the famous space-cadet keyboard of 1978):
Now, I'm not saying that such Shift keys were ubiquitous in the past, nor that they are never found at all on modern-day keyboards, though in my experience they were much more common thirty or forty years ago than they are now. I'd like to know the reason that these Shift keys (and no other keys on the keyboard) were designed this way and why this design seems to have disappeared. Was there a particular ergonomic reason for the shape that applied back then but that is no longer relevant today?
I suppose it could be argued that the unusual shape makes the keys easier to distinguish when touch-typing, though I've personally never had any problems finding the Shift key when my fingers are on the home row, and besides, the recessed edges seem to make the key harder to strike reliably. Moreover, ergonomic aids don't seem to be high up on the list of features of ancient keyboards—for example, I don't recall seeing any of them with bumps on the F and J (or D and K) keys that are used today to help touch-typists find the home row. If there really is some ergonomic benefit to this Shift key shape, then why don't most modern keyboards (even relatively expensive ones) continue to use it?
Is it possible that this Shift key design had its roots in mechanical typewriters, and got mindlessly carried over to computer keyboards? If so, what mechanical benefit did this design have, and why was it applied only to the Shift keys and not to other large keys such as the Return key or (on some typewriters) the Tab or Backspace keys?