I to am a machinist...
If I were to do that job it would be nice to have a slotter, basically a vertical shaper with a built in rotary table.

From what I can see in the pictures I would say he used the quill of the milling machine as a slotter with a stationary tool to cut the splines, advancing the table slightly for depth of cut, never even turning the mill on.

It is nice work!

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-