Let me mention a typical scenario that could explain the situation as it has occurred many times to my knowledge in the UK- and probably in many other Armourers shops throughout the world.
85 various No4, No8, No5, Stens, Sterlings, L1A1's (notorious.....) etc etc go from the Ordnance Depot or a unit for some reason, into the main workshops for a total overhaul/rebuild. During the rebuild, following a complete, every last nut and bolt strip down and examination of every part, the slow assembly takes place. At the final out-inspection stage, there are 85 like brand new rifles but because of the actual bead blasting process, the phosphating and hard oven bake painting, 7 of them don't have easily identyifiable serial numbers. So the under worked R&I clerk double checks the paperwork against the rifles ready to go out and identifies the serial numbers missing from the rifles. He then send a note back to the In Inspector at the Armourers workshop that tells him that these 7 rifles are missing OR have unidentifiable serial numbers. But don't worry, the serial numbers, identified from the paperwork OR the unit WOCS books are these, xxxxx, xxxxx, xxxxx and so on.
So a supplementary job card/number is allocated and the Armourer will renumber each of the rifles with one of the numbers on the list. So 85 rifles come in and 85 fully identifiable rifles go out............... Naughty but that's a fact of how life works at the sharp end!
The alternative is for some form of fraud in its past
Some of the real experts will chime in and give other plausible (and some total rubbish too I expect......) explanations but that's my experience.