i bought one from Waffenmeister. Works fine so far. I had Deerslayer convert it to a '21 and what a joy it is to shoot now!
https://www.waffenmeisters.com/thompson-bolt-actuator-1928-smg-p-327-1?search=thompson
I thought I lost mine few years ago. Found them in place I don't recall putting them.
Lesson learned. I scanned them into .pdf files, made copies and put the re-discovered forms in the safe.
I remember there was a page in one of the manuals that actually showed how to use the actuator and front sight to engage a target.
Don't have the manual handy right now.