For the 30s and 40s, in the archives the most common forms of reproduction I see from general office records is carbon copies and spirit duplication, aka the Ditto Machine. Those are those purple copies with that ever so lovely smell when they’re fresh, though if you’re asking this you might not be old enough to remember that! :) Here’s a video of a machine in action. These can make one copy or many, so they were popular in offices I guess because you often only need one or two copies. Mimeographs were around but are slightly different, since they had to have a master stencil, so they weren’t as good for general office record keeping. They were better for making small runs of things. I don’t see them as much.

Carbon copies are pretty old. You made a carbon-paper sandwich - paper, carbon paper, paper, then you load the sandwich into the typewriter, or just write on it with a pen. The ink of the pen/typerwriter ribbon makes the “original” copy, the pressure of the pen would transfer the carbon paper ink (which is dry, like typewriter ribbon) to the paper. (Nota bene: doesn’t work with conventional fountain pens as they don’t have enough pressure!) It only makes one copy. This was used to keep copies of outgoing mail in offices, among other things. I actually can’t find a decent video of carbon copying! But if you’ve written a check you probably have the jist of it.