Actually maybe/probably not. The guy in the video
is using a vertical milling machine , eg.
View attachment 1011537
I have heard that is that this type of mill was invented in the late 19th cent. for rifling gun barrels, personally I'm not so sure about this (was invented late 19th.C.) but I do know that the more standard milling machine in the early 2oth.C. , esp in heavy industry, was the horizontal type. eg's
View attachment 1011538
Seen set up for gear cutting.
View attachment 1011540
Both types were used side by side in industry for a very long time & it is only in the last 40-50yrs (that makes me feel old) that the vertical has gained supremacy.
Which type was used for this exact job - I don't know. Given we're talking 1915 my money would be on the horizontal.