These have nothing in common with a Thompson gun except a very vague resemblance if you squint your eyes.
These were made by two companies, Volunteer Enterprises made the gun from 1969 to 1976.
Some people like them, many people had problems. The only way to tell the difference is to shoot the gun a few hundred rounds.
These use M3 Grease gun magazines.
Value depends on condition. Guns with the vertical pistol grip forward grip are worth 10% more.
The Blue Book lists values as: