Peter makes a valuable point about the "barred out" markings.
In Commonwealth service, it is standard practice to cancel a marking (serial number, designation, etc.) by engraving a one or two fine(ish) lines through the text, instead of scrubbing / machining it off completely.
The reason is so that the ABSOLUTE history of the accountable component, in this case, the receiver or body, can be tracked.
This happens a LOT with machine-gun barrels, for instance when they are down-graded from "live-firing" to "blank-firing" because of throat erosion, muzzle damage, fail accuracy test, etc. Sometimes you will see "live" MG barrels with a number barred out and a new one engraved. This usually indicates that the BODY of the MG has either failed a major inspection and been scrapped as "un-repairable" of been "lost" in action /on exercise" etc.. And then there are “operational incidents” on the two-way rifle range……….
MGs and all manner of other weapons and expensive equipment have been known to fall out of helicopters and over the sides of ships and boats of all sizes, get smashed up in vehicle accidents or burnt in warehouse fires etc., hence "orphan" parts.
Thus, the need to re-number "key components" as they are "recycled", but still be able to read the previous number(s): all part of the procedure of "tracking" critical / accountable items.