I have a colt upper that appears to have a compensator on it instead of a FS.  It has 5 cuts to the upper half and is solid on the lower half.  No bayonet lug on it either.  

First was this common for Colt to do this to PreBan rifles?  Is this a comp or a flash suppressor?  I would like to put this upper on a post ban lower.  Would this be legal?

Any help would be appreciated.

Thanks

Generally, if the muzzle device is "open" at the front, it's a flash supressor, but if it has an opening only slightly larger than the exiting bullet, it's a compensator. Some pre-ban Colts did have the A2 flash supressor, but came without a bayonet lug.  Don't ask me why, some idiot at Colt dreamed up that one.

If it IS a flash supressor, it would have to be removed and replaced with a permanantly installed compensator, or the barrel threads removed to be legally installed onto a post-ban lower.

It sounds like it is a pre ban colt upper, they neutered the bayonet lug back in the late 80s, early 90s. If it is a real unmodified Colt upper, and post ban, the comp should permantely attached-but wait I don't they offer muzzle brake/comp from the factory. Sounds like a pre ban with A2 FS. Bottom line is that if what ever is attached to the muzzle is easily removeable, and there is threads underneath it, no doubt yo have a pre ban. See if there is a light colored washer between the muzzle device, and the barrel. If so, this a peel washer, or crush washer that is used to index the unopened side of the FS downward, hence preban.