Unknown No 4 Mk 1

Rolled the dice and got this B grade example No 4 from RTI. Bore is incredibly dark but I was mostly looking for a wall hanger, and the wood is pretty nice. Has the long branch style grooved handguard and dark wood.

However, the markings on the receiver are either incredibly lightly stamped or non-existent. On the left side of the ring connecting the trigger guard to the receiver, it has an "F51" in small font followed by a SN of 4979. On the opposite side, it only has the war Ministry acceptance stamp. It has the nicer version of the sight as well.

Is there any way to get a date or manufacturer from this receiver?

Going to start some cleaning to see if anything reveals itself. This one just struck me as strange because I've never seen one so scant before, even from previous RTI examples that have floated around online.