For old calculating machines I want to recommend Prof. Chris Staecker's YouTube channel, his videos are enjoyable. Here is a play-list:

https://www.youtube.com/watch?v=swCy6pQSEpk&list=PLLFpXNanTP...

Inspired me to buy a Comptometer:

https://en.wikipedia.org/wiki/Comptometer

If all you need to do is add, Comptometers are the fastest machines so far invented. They are fast because you can enter entire numbers at once (as a "chord" with multiple fingers pressing multiple digits at once). No need to hit "+" key or pull a lever between numbers, each button press is an immediate addition.

I'm thinking of making an electronic version- it would have the same Comptometer key layout, but have more operations (for example, hold multiply "shift" key down while entering number for a multiply).