It gets stranger:

If that is an "R" prefix on the serial number, it shouldn't be a Lithgowicon produced rifle.

To the best of my knowledge, Lithgow serial numbers ran from "A" series to "F" prefix; 1944-45 and the last run in 1953. The "B" series ran from 1922 to about mid 1941.

Furthermore, there appears to be a Britishicon proof (Crown, GR, crossed pennants and a P) on the left side of the knox form photo. Lithgow barrels of the time would have had either the LP and crossed pennants or "ACP" in a shield, depending on date.

I am wondering it it may be one of the many Brit manufactured jobs that found their way back to Oz from sundry battlefields after WW1 and was put through the mill in mid to late 1941. Original serial number, barrel and possibly bolt body retained but lots of Oz replacement parts.

Check the Left side of the receiver socket for any signs of remnants of other markings.