The Broomhandles, Lugers, and High Powers were specifically exempt by the BATF. A copy of any of those pistols fabricated by an individual is not, regardless of the serial number. John Martz made copies of the Luger carbine with a shoulder stock, but had to make the barrels 16" long to be legal.
Not all Lugers are legal with a shoulder stock, but specifically the WWI LP.08 (Artillery) and Navy, and the commercial Luger carbine are exempt as a group whether the stock serial number matches or not. On any of the mentioned Lugers a reproduction stock is also legal.