Nice tear down.
I remember seeing the first HP 35 calculator and quickly acquired the TI SR50 scientific calculator that I used through my college classes and on the job.
Now if you want a real challenge, see if you can get access to an old Friedman mechanical calculator.
We had one in the office in 1973 and it was amazing to see it solve problems.
Why so fussy about the power supply for the desk-calculator? Just use a step-down or an (isolation)-variable transformer, the difference between 50/60 Hz can't really mean anything to this kind of device, as it doesn't rely on the mains frequency to spin a motor at a constant speed, like clocks, tape deck, record players and so.