A significant change was the transfer porting and breech arrangement. The mark 1s have a brass bush loctited to the end of the barrel; the breech seal groove is machined into this bush, and the transfer port also drilled into it.
It was done this way as AA didn't want to have to run the risk of machining directly the steel barrel, reckoning that any scrappers would mean a lost barrel, whereas a scrap brass bush was only pennies compared.
The bush system did require perfect concentricity which wasn't always the case, and some barrels could end up with the hole in the bush not perfectly aligned with the bore of the barrel.
For the mark 2s and later, they negotiated with the barrel suppliers that the suppliers would do all the drilling and AA would receive ready-to-fit barrels. This put the risk with the supplier, and meant the brass bush was redundant.