Since you're based in Australia, I wouldn't rule out that this is a gun registration number added later in it's life when rifle registration was adopted in England or when it first arrived in Australia. I have an 8x56 Mannlicher-Schoenauer factory dated 1925 with a very similar engraved number, A1525, on the trigger guard and also repeated on the bottom of the scope body. The rifle is stamped "Not English Make", so it was imported into England soon after manufacture. However the name, S.L. Rohu, Gunmaker, Sydney, appeared on the original sling. I purchased it in New Zealand, so it's been around.

One of the arguments against registration of firearms is that they'd be defaced by putting extra numbers or even bar codes on them. One of those would look good on a classic Mannlicher, eh? Authorities deliberately avoid hiring people with any affinity to gun owners, which explains why they have several hundred Winchesters on such records in Australia with the same serial number M1894 and similar. This is, of course, the model number and the real serial number is on the other side. This sort of explains why they couldn't just accept the maker's serial number and had to stamp or engrave their own.

I took my .360 No 5 rook rifle serial number and asked the A&N record keepers about it. They told me it was a 20 gauge shotgun. Go figure.