Hmmm ... afraid I can't say for sure. I put in the Jap spec box along with the import 4age and Jap spec rear disc end so technically it all genuine Jap spec items. As I didn't leave the existing Aus spec T-50 box, I guess I avoided any problems which could arise.
Still, there is nothing wrong with the box & it was working fine when it came out. I'm hanging onto it as a spare but won't mind separating with it if the price is right & given that I have two now.
I also thought that the gear ratios were the same in both boxes but I've since found that my speedo seems to be overstating my speed. Perhaps I struck it lucky by getting a box with close gear ratios? Ooer ...