US Classics - How Were Plate Numbers Entered?

The image shows Scott 698; the plate was made in 1933. For issues in this period, how were plate numbers actually entered into the plate? Was there a numbered die used to enter each digit? Were the numbers hand cut into the plate? Was a plate numbered slug attached to the plate? The basis of these questions is a "problem plate number," showing number alignment errors and misprinting, discussed in 88 The United States Specialist 12 (December 2017) at 551.



thanks,
Chris