The factory MR556 barrel extensions have the proprietary tab for the carrier cut out (easily milled off by your smith), and the OTB nub on the inside to save the extractor. A few of these were offered very briefly on Brownells, and haven't been available for quite some time. And after looking at my records, they weren't cheap then either.
I deal for you HK re-barrel project
Paypal gift (or you pay 3%) or USPS money order.