Wolff has documented early mechanical, electromechanical, and fully electronic calculators. He also has a smaller collection looking at music boxes.

Of particular interest is this exceptially detailed breakdown of a Marchant "Figurematic" Model 10DRX, here: https://www.johnwolff.au/calculators/Tech/MarchantDRX/Intro....

If you, like me, have recently become obsessed with (electro-)mechanical calculators, here are some videos of their operation: - https://www.youtube.com/watch?v=axtQvkSpoto (Friden STW10) - https://www.youtube.com/watch?v=fe-9uTVMQlo (Comptometer) - https://www.youtube.com/watch?v=cT8fSGHHiAs (Addiator/Addifix, a pocket mechanical calculator!).