When does a Luger Stock make it an SBR?

[Edward Tinker]

There are exceptions - don't guess, google ATF regulation 5300.11

but since i wanted to double check >>> https://www.atf.gov/firearms/curios-relics

then do a search for luger

A few years back the Finnish Luger was placed on there, 4 inch, 7.65mm or 9mm and a stock. Pretty open interpretation....

Finnish, m/23 Deutches Waffen Munitionsfabriken (DWM)-manufactured Luger pistols with 95cm or
120cm barrels in 7.65mm or 9mm calibers, accompanied by a German- or Finnish-manufactured
wooden board shoulder stock.