The metal has to be displaced whatever speed it is done at. I would guess that a fixture was used for the US SA stamp with a hand lever toggle set up to get enough pressure. I have used and built air toggle presses and you probably use hand operated toggle press to reload. The serial number could have had a stamp holder with replaceable elements or rotating wheels. It may have been semi automatic as it would be difficult to number receivers all day long without using the same number multiple times and skipping others. When you have done production machining for 40+ years it is easy to visualize all the things that can and do go wrong.