My take on it...the 27's had a removable stock , by WWII and the A1 the stock was screwed on ( a war time cost concern )
which also deleted the drum Mag mounts in leu of sticks.
AO (West Hurley) produced Model 1927A5 Semi Auto Thompson Pistol (mine)
( as does Karr ) a model that is considered a pistol " 1927A-1, Deluxe Pistol "
Apparently. there is an adaptor to add the butt a to make an SBR
I'm just getting into to it as to options...I need some stick mags first , I may SBR it at some point , if the current Fed's don't lock it out