Am I missing something!

[Bruce_in_Oz]

For those in the anorak seats, I found the included angle of the thread on a No1 Bolt head:
79 degrees, 30 Minutes. It is depicted as a triangular thread with notes and tolerances for the form of the root and crest. There is actually a "Go" and "No Go" gauge, both of which are "adjustable to master "plug" gauges by means of a (lockable) screw, not unlike as found on thread dies. NO CMM gizmos in those days.

This is taken from Lithgow drawing C 4983, dated 20 October 1944 which has, hand-written across it: "Superseded. See Data Sheet DS 464, Sheet 410-3-52". This was, in turn, a revision of a 1935 version of the drawing.

Thus, Lithgow fine-tuned some gauging in the process of doing the last ever run of No.1 Mklll* rifles at the factory.

Sadly, i do NOT have this later drawing in the "crypt".