cuervo

The original AR15 did not have a forward assist; this is a Colt SP1 and they can be pricey since they're old. The A1 added a forward assist, which can be round or tear-drop shaped. Early civilian AR15s may have the original open flash suppressor or the birdcage. Later civilian AR15s switched to the birdcage since that is what was in production. Both of these used a 1:12" thin barrel, which are getting harder to find since production shifted to the heavy barrels back with the A2 uppers. Fulton-armory has them for $199 (just the barrel) and can build you a true A1 upper if you want.

Finding an upper based on an A1 receiver is pretty easy since many people prefer the adjustable sights of the A2. Finding one in an original configuration with the pencil barrel will be a little harder, but they're out there. You can also try searching on auctionarms, gunbroker, or gunsamerica.