I actually used one for a University assignment
This was in 1979 or 1980 (I can't remember exactly - getting old).
Was my first experience of 6502 machine code, which became very useful when I got my BEEB a few years later. Had to code a sine-wave generator using an attached D-A converter. Real pain putting the opcodes directly into the keyboard, with no means of storing the program.
I think that this one must be a later one, because the ones that Durham had not only used a calculator keyboard, but also a tiny calculator display as well, mounted in what I remember to be the top half of a Commodore 8 digit calculator. My memory may be playing up though...