Please post pictures. Follow the guidelines shown elsewhere on this site.
Back in the 1920's, several companies were importing Lugers to the USA.
Some of them offered a variety of lengths of barrels.
Depending on several details, your gun could have originally been a Luger Carbine, with the shorter barrel swapped/added after the fact. <From photos below, this is not the case...>
In any case, pictures are pretty necessary for any kind of analysis.
Marc
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 Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.
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