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On those old receivers none of the internal cuts were done by forging or casting them in. They were all machined into rough forgings. The locking lug seats were cut in different ways. In the M1903, they were machined by an end cutter, since the barrel was not installed and the OD of the lug seats was the same as the ID of the barrel hole. The cam angles were then shaved using a special tool.
In the Mauser 98, which uses an internal collar, the cuts were made by a using a rotary cutter that was offset in the hole in the receiver. (The same technique was used to cut the locking lug seats in the 1911 pistol slide, which is why a barrel bushing had to be used to fill the oversize hole in the front of the slide that was needed to admit the cutting tool.)
I apologize for my early error in saying a broach was used; for some reason I was thinking of the bolt lug raceways, not the lug seats.
Jim
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