It might be possible you have a J. Jovino imported receiver. if you look under the charge bridge (right side) there may be faint "JJco" import stamp. They are a very small font, and almost imperceptible to see...
John Jovino Company imported many AustralianSMLE Mk III's in the early 1990's. It was theorized the company bought most if not all of the remaining SMLE stores, including new unmarked stock sets and un-numbered receivers + barrels. These were assembled into rifles, given a serial number which do not fall into the known serial number chronology of know Australian rifles, and fitted with brand new coachwood stock sets. They were given a very dark, almost black finish and advertised as "unissued" rifles.
Short answer, your receiver might be one of the Jovino parts group, and simply never assembled.
If it were mine, and I had a pile of money to burn () I would assemble it with a new Criterion barrel and a Prestigious Wood Stocks set. It's nice to dream, right?
Post a few pictures if you can. Be interesting to see what exactly you have.
*edit-managed to find two pics of a J. Jovino import stamp on two of my rifles. Told you it was small font!