I've eaten C-Rats dated 1971 as late as 1985 but we never used a piece of equipment like this. Anyone familiar with it? The wingnut is stamped PAM on the nut and " EF " on one side of the wing. There are index marks for resetting the front and rear components. I see what I think is iridium paint on the sights (inset) for night use. The wing nut obviously clamps the apparatus onto a tube or barrel (I think) but there appears to be no locking mechanism or recess for a lug or provision for sliding over one. I thought it might be a type of FAL grenade launcher but the markings make me think U.S. Any vets that served during 'Nam know what this goes to?