Another data point if anyone’s interested. I bought a Carbine marked CR6700A4 last year. I recently bought a real deal Colt M16A4 upper off EE and wanted an AR-15A4 marked lower to throw it on. I ran into a new AR15A4 today locally and picked it up. Here are my observations about differences between the two. Got the Carbine marked one last year from GT because they were cheaper and I had the opportunity to inspect at the time.
Lower
Rollmark is the obvious difference. Won’t rehash that. The A4 today has the shiny “8” marked grip, and the new style ambi selector (non S marked but still reversible...Colt short side lever). Carbine had the S marked hotdog style ambi selector, same short lever, and the chalky grip. Both had selector stops. Interestingly, the receiver extension on the A4 is the dull gray variant. Carbine marked is the shinier black. No variation in stock body. Trap doors are the same (all polymer).
Upper
Barrels appear the same. No marking other than C M P 5.56 1/7. Handguards are the same. Good quality...glue where the heat shield meets the tabs. Receiver on the Carbine is rollmarked with the cage code. Sadly, the A4 is not. Both are designated M4 where the gas tube enters and have white T marks. Both dry film treated on the inside. The carrier in both are dark black and marked (etched) C. The bolt in the carbine is faintly marked MPC with an A underneath it. On the A4, it’s marked MPC but you have to really look hard to see it. Of note, the barrel on the A4 has the yellow paint mark by the gas block. The bolt had the white grease mark. Neither were present on the carbine.
I’m pleased to have found an A4 marked variant and not have to order it. Do the differences mean anything? Who knows. Probably mean they were both built with what was in the parts bin that day. I’ll probably source a chalky grip for the new one. The new upper receiver group will get set aside for now, so I’m not losing sleep over no cage code, although I would have preferred it. This purchase was about the lower.