The A6 came out during the Italianicon campain right before Rome fell IIRC. The WW2 style handle was a round tube with a flat plate with a hole for the shroud at each end. It slid up and down the jacket and was too far forward to be balanced . The BAR handle came out at the war's end or just after. They used it as a model for the later A6 handle , replacing the bbl bracket with one that fit the A6 jacket. It could be bolted tight in one place and if all the way back with the handle over the reciever you can get a fair balance .
I think the A4 bbl will work in the A6 setup , but I need to try it before I can be sure .
I have samples of most of the USGI WW2 beltfeds , otherwise I would not know about these things either . :>)
Chris