I am a BritishArmy Armourer and I can assure you that contrary to popular belief, we didn't go trawling through trays of parts just to find a LB or S or any other matching part and don't believe anyone who tells you otherwise. 9 rifles held in the battalion came with 9 transit chests and until they were painted up to match the rifle to be issued to an individual sniper, it was just another transit chest - and they all fitted!
If a mazak(a sort of sintered alloy) butt plate snapped/cracked across the heel screw, which they were notorious for and regularly did, then it was replaced with - have a guess.............., yep, got it in one - the next one in the tray. And if it was brass, that's what it got. Mind you, fitting a new butt plate was another of those hand fitting jobs to do properly. Making sure that it fitted correctly underneath (yes, we did.....) and along the sides
I'd say that from your description, your rifle is as original as the day it left service