I bought my first AR at a gunshow in California about 1985. It had a collapsible stock, short barrel & long flash-hider. After awhile, I noticed that it didn't look like all the other "CAR-15" flash hiders, and in fact, looked exactly like the XM177 flash suppressor in my Military Weapons book, right down to the little collar at the back of the flash suppressor. I started wondering and stuck a cleaning rod down the barrel and measured the barrel length. 15.5". Thats when I got nervous and ended up selling it off. I can't say it was an original XM177 suppressor, but it could have been. I know that a number were out on the market before ATF ruled them to be a Class 3 device because they also moderated the sound to a degree. They were then supposed to all be registered and serial numbered, but I'm sure some slipped by. Sometimes I wonder where it ended up.
This was a topic of discussion on another thread, but it also had an M16 trigger, hammer, disconnector, selector and bolt carrier. It just had a little pin sticking out of the side that prevented the selector from moving to the auto position.