Maybe I'm wrong but I think this answers your question.
"Our Firearms Classification Panel has examined your request and is their opinion that the above mentioned pistols equipped with currently made reproduction shoulder stocks which either duplicate or closely approximate the dimensions and configuration of the original stocks would also be primarily of interest to collectors and not likely be used as weapons. Therefore, any Luger or Browning Hi-Power pistol which would be removed from the purview of the NFA if equipped with an original shoulder stock, would also not be subject to the NFA is equipped with a reproduction shoulder stock which either duplicates or closely approximates the dimensions and configuration of the original stock."
http://www.empirearms.com/curios.htm