[TDR]

I am not sure that this would be a valid reason for not including this gun in the ANZAC OOB.
Also why would it need a new class/formation? It is just another AT gun like the 2 Pounder.
As for the Japanese players, as I pointed out it was in PNG in 1943 anyway so I would see little grounds for complaint on that as well.

Ref 1.
Australian War memorial, picture ID# 024807
Caption: "WESTERN DESERT, EGYPT. 1942-08-10. MEN OF 3RD AUSTRALIAN ANTI-TANK REGIMENT AT FIRING PRACTICE ON A 6-POUNDER ANTI-TANK GUN ON THE COAST AT EL ALAMEIN."

Ref 2.
Australian War memorial, picture ID# 060433
Caption: "GODOWA, NEW GUINEA. 1943-11-05. GUNNERS OF THE 11TH BATTERY, 2/3RD AUSTRALIAN TANK ATTACK REGIMENT MANNING A 6-POUNDER GUN AT SIMBANG, LANGEMAK BAY."

Of note between 42 and 43 the Anti tank units had a name change from Anti Tank to Tank Attack.

This seems the gun should be part of the ANZAC OOB not a buy from the UK OOB.

The 6 Pounder Anti Tank Gun was being manufactured in Australia as was the 17 Pounder Anti Tank Gun.

Ref 3
Australian War memorial, picture ID# 029419
Caption: "FORT GELLIBRAND, WILLIAMSTOWN, VIC. 1943-03-02. AN ALL-AUSTRALIAN MADE 17-POUNDER ANTI-TANK GUN UNDERGOING TRIALS."

edit:
Also under the ANZAC OOB thread in this forum I have listed a few units/weapons with odd dates.