As far as I know, the electo-pencilled markings are an Australianicon thing, often accompanied by two, roughly drawn crossed pennants. It is a proof mark.

It seems to be found on replacement bolt-heads, because the one in the rifle is essentially "proofed" with the rest of the barreled action at the factory.

I haven't found it, yet, but there is probably a clear set of instructions somewhere.

Interesting note on the Lithgow drawing for the bolt-head, C-643, dated 13January 1921, is as follows:

NOTE! For spares, qualify between 9deg and 13deg in advance of new component. Thus ALL replacement bolt-heads as first issued, have a built-in allowance for the gun-plumber to "fit " it to an already fitted bolt body and/or allow for thread-wear inside that bolt body.

The only marking specified on that 1921 drawing is the letter "M", which is to indicate bolt-heads made from "mild steel" as opposed to the originally specified "malleable cast iron".

Further digging required. I can't be the only keeper of old Lee Enfield tech docs on the planet.