Don't think that there's anything totally unusual about serial numbers being overstruck. We'd do it if the need arose. I saw a No4T not long ago with an overstruck BSA number of R-33XXXX that was clearly (or not so clearly) but originally R-32XXX.
If you COULD identify an indistinguishable or illegible number using the master/QM's WOCS list then all the better otherwise you had to get a new number allocated from a special security office somewhere and these were in the ZZXXXX or SA-year- AXXXX range and that really was a pain!
I know I blame them for everything, but Fazakerley were always the worst offenders with their No4's, Sten Mk2's and 5's and Sterling L2's. Mind you, BSA No4's weren't masters of perfection.....