It's a brand new Maxi-Switch MAX II keyboard, made about 1979. From the press release, https://www.computerwoche.de/a/max-tast ... ms,1193136, poorly translated out of German by myself:
The MAX II has the same form factor as the MAX I, also are the plugs compatible. The keyboard works with a Intel-Microcomputer 8048 (or the programmable 8748), the scanning-signal for the 60 keys together generating the corresponding US-ASCII codes. For each squeezed key the corresponding code strikes the exit in parallel or in RS232 serial. Because of that the possibilities of this keyboard --- cry [the manufacturers] --- will be labelled as intelligent.
Essentially it's an industrial keyboard which generates ASCII. (Mine doesn't have the serial attachment.)
Embarrassingly, it took me until I got it home and unwrapped it before I realised that the layout was a bit familiar; it's the spitting image of a BBC Micro keyboard. The only difference is that the backtick/pound sign code is generated by shift-@ rather than shift-underscore.
This suggests that the BBC MIcro layout wasn't made up by Acorn --- it's a copy of an existing mainframe layout. This keyboard was made in 1979ish, at which time Acorn was working on the System 3 and the Atom, which had a related but different layout. This suggests that someone had exposure to whichever mainframe my keyboard is copying while doing the BBC/Proton industrial design. And I've also found some very similar Happy Hacking layouts, which corroborates this: http://xahlee.info/kbd/i/happy_hacking_ ... 447832.jpg
So I reckon this is a standard, if an obscure one. Anyone know where this came from? If the layout originally came from a mainframe terminal keyboard, it'd be interesting to know how the Acorn engineers got exposure to it...
(Well, I'd find it interesting.)