The last production 351C engines (both 2V and 4V versions) were assembled in the US circa June 1974. 351C engines with 4V cylinder heads were never assembled in Australia. Australian heads do not have a "2" or a "4" cast in their corners.
4-dot cylinder heads were cast in the US for M code engines manufactured from July 1970 through June 1971 (D1AE head castings), and Q code engines manufactured from July 1971 through June 1974 (D1ZE and D3ZE head castings). However there was an extra run of D1AE-GA head castings produced for the NASCAR teams in about the time frame of 1975/1976, the same time frame in which the first batch of Australian sourced NASCAR blocks were also manufactured. A 4-dot head with a December 1975 casting date would almost certainly be one of those D1AE-GA castings, having 66cc quench combustion chambers.
Heads from this batch of castings were not installed on any production engine, thus these heads were added to this short block after the fact; no doubt the short block was originally equipped with open chamber heads.