when was the recessed circular cut added to the elevation serrations

[RCS]

Back in 2015, i did some research on early M1 receivers from the 1941 period (this also includes Winchester) in an attempt to find the modification to the elevation serrations which are machined into the receiver on the left side. Early receivers I called the flat side and have found Springfield serial number 187106 as the highest flat side. On Winchester I found WRA 103855 as the highest flat side.

Springfield s/n 195862 has the recessed circular cut under the elevation serrations and Winchester 104298 also has the recessed circular cut too. It is possible with Winchester to find a single odd-ball
serial number higher without the circular cut.

Interesting to note that both Springfield and Winchester added the circular cut to their production receivers around the same time.

Also found that the radius cut was added to drawing under revison -3 21 June 1940. Tolerance was changed on 8/18/40 under revision 5

The circular radius cut was added to the drawing on 21 June 1940

[Jersey Devil]

Great info, as always, Robert.
Thanks for sharing your research with us.

[RDS]

Robert why was the cut made? What purpose does it serve.

[TJT]

From a (retired) machinist's point of view, I would opine that it was done to assure a perfectly squared and perpindicular surface to the pinion just in case there was some kind of minor defect (swale or taper) on the side of the receiver from a previous operation. It assures the serrations are "all there" and of a uniform dimension.
Just a WAG on my part.
Jon.