I see that over on the FES forum its been put forward that the 1 stamp denote its a 1st class arm.
I have a 1918 Lithgowthat has a 4 stamped there, is mine a 4th class arm??
If your rifle is stamped in a similar way, the 4th Military District was South Australianissue.
A 4th class arm, reckon that would be well and truly ZF'ed.