You won't be able to find an A1 back(pretty sure they didn't exist), but you will be able to find a G7 fly threaded back. Same threading, just a longer shank. Throw a beauty ring on it, or leave it be.
Stanchy has done several A1 backs. Obviously you want to use the largest bore back for the donor (sticking your .691 insert into a back that was originally .689, for example, obviously defeats the purpose since the A1 back is longer than the Freak insert - you'll be stepping down bores from the insert to the barrel). I've heard nothing but good reviews from the people who've had theirs done.