[HerrKaiser]

BATFE rules that Artillery Lugers with original manufactured, and appropriate, stocks are 100% exempt from NFA '34 restrictions. (I.E., a military stock for a military Arty, a Navy Stock for a Navy Luger, and appropriate commercial stocks for commercial artys/navies/other Commercial variants as listed on BATFE website.) So you cannot slap a navy stock to an arty Luger or it is an SBR needing the $200 tax stamp and paperwork.

Several members here present letters from the BATFE regarding this question that show "close"/"faithful" reproduction stocks are also okay as the guns in question are "primarily collector's items unlikely to be used in a crime." Its a bit of a gray area with repros, and its not codified in the BATFE official document like the original stocks are. So the above letters could be theoretically overruled on a whim by the agent you ask this time and it wouldn't take much from a legal standpoint to uphold that decision. Best of luck!