Colin,
Tanks were normally purchased with a suite of spares, particularly fast wearing spares like wheel assemblies (the tyres chew out). It is rare indeed to have a tank survive its service life with the original 'when assembled' set of road wheels - normally these are changed as required. Spares were not always supplied 'up front' but on a yearly basis depending upon the terms of the contract. So it would not be unusual to have wheels manufactured and supplied after the tanks entered service.
In the case of the Vickers Mk6A, these were built as standard - I don't think there was any particular 'Aust standard' for that small batch - and they arrived circa Sept 1937, not 1939.
Wheels supplied as spares could have a number of production dates, and therefore those on the tank now could well have a range of production dates. It will be interesting to see what other dates/production details become evident as you blast the crud off the rest.
Mike
