Well - I'm going to vote for Calculator - though I learned to program on
one of these back in the early 70s. I wrote a couple of simulators for
them to learn Fortran - and still remember a few details.
1) The machine had a total of 120 instructions if you used up the upper
registers with instructions (so it WAS possible to carefully do self
modifying code if you were so daring.)
2) When you used needed those registers - it would be 60 instructions.
The machine was very useful as a training vehicle for preparation to run
into things like the Altair with 256 bytes, etc. I look upon the P101
with a lot of fondness!
Steve