Thank you for posting link. Learn something new every day. I would guess that most rebuilds would have been done at base workshop in BAOR or at the Workshops in Warminster, sorry can’t remember names or numbers, though no evidence to support this.

The FTR engraving looks very similar, in depth, style etc to the numbering which would of have been done at manufacture, doesn’t explain anything though. On Enfields the FTR mark was usually identifiably different to the original manufacturers markings, does not prove anything in this case though. On the L1A1 the master number was on the body.

A couple of unsolicited observations re this rifle, based on the photos.
The body hinge pin is in backwards, is this affecting the safety function of the weapon?
The pin for the flesh eliminator key is missing. Is the flash eliminator fitted correctly?
Hand guard screw in backwards, for a Britishicon weapon any way, might explain cracked hand guards.
Needs a dam good clean, with this level of care what else has the owner neglected to do?
The breech block should be serial numbered to the rifle, I am unable to see any number at all.
The serial number on the carrier does not match anything.
Whilst the add does say it is a build up from various bits, the lack of matching serial numbers is a real big red ***. Gauging will be critical to ensure safety prior to any firing. Good bargaining chip.
From the photos I would say that this rifle has been assembled by someone who really doesn’t know what they are doing or doesn’t care.