I don't know about the current Thompson semi-autos, but the older Numrich Arms and Kahr Arms guns had threaded muzzles.
On the older Numrich guns the compensator was actually GLUED ON with epoxy, and you had to warm the metal to 300 degrees to get it to break down so the compensator could be screwed off.
At least the early Kahr Arms guns were still treaded, but they used the original Auto Ordnance SMG method of a threaded muzzle and a cross pin to lock it in place.
Before attempting to get the compensator off, FIND OUT WHAT YOU HAVE.
Trying to pull off a threaded compensator isn't going to work.
The Thompson M1 and M1A1 type guns with NO compensator had smooth belled muzzles and a cross pin. The guns with a Cutts compensator are/were threaded and either epoxied or pinned.