"M" bolt head

[Bruce_in_Oz]

Sadly, I have nothing further regarding service "life". Lithgow seemed to have thoroughly committed to the "M" version, (starting date unknown to me) and stuck with it to the end. Both types are prone to wear at the little "bump" / "hook" that runs in the groove on the RHS of the body, but this takes a LOT of "cycles" of the action and lifting the bolt head past the "Spring, retaining" every time the bolt is removed for cleaning, inspection, etc. .

One reference for the "M" (or lack thereof), is: Drawing no. A.I.D. 1529, which is a copy of the earlier R.S.A.F. 3095(1). This accompanied Specification S.A. 1117 (B) / R.S.A.F 3095 (1).
Viz:

Material: Wrought Iron or Mild Steel. Hardened and Polished.

Lithgow drawing C-643, (13January, 1921), has a bit more information:

1. Note: For Spares, qualify between 9deg and 13deg in advance of new components. (Thread is "advanced" to allow for wear of the thread in bolt bodies).

Thus, Lithgow DID make different bolt heads, "original fitting" ones and ones that had a built-in wear allowance. How these "spares" were to be identified in packaging is NOT noted on the drawings. The "tolerancing" on the length of the "cylindrical" part of the "factory new" bolt head is: .635" Accept, .632" Reject. (effectively, less than three thou). HOWEVER, post WW2, they produced and the "system" introduced EMEI paperwork for, a "range" of bolt heads in "one thou" length increments. This was to "tide them over" until the complete adoption of a "new" rifle, i.e., the L1A1.

2. "Face of Bolt Head and Hook only to be case-hardened to a Minimum depth of .01" "

Further note, with an arrow pointing to the top of the "lug": "Mild steel bolt heads to be marked "M" here".

Still ferreting for any additional information.