Jeremy: I believe you are basically correct.

IIRC - the SAAMI dimensions moved the 7X57mm case shoulder back slightly, which created a potential excessive head-space situation in Mausers and Rolling-Blocks with the original dimensioned chambers.

If new Brass is fitted to the early chamber, by 'Fire-Forming' or using lower pressure 'starting' loads, and the cases are neck-sized only, when reloading, Life should be Good!

(p.s. Try chambering cartridge cases fired in your FN-49 to see if they will fit in your R-B chamber. If they do, it is worth neck-sizing them for use in your Remington R-B. Just back off your F.L. Resizing Die a turn. Chances are, the case shoulder moved forward a bit when fired in the FN-49).