Boomer, good point. Yeah, I believe when the military first wanted to design an issuable M16 carbine, they basically put a Car stock and a shorter barrel on a standard M16A2. Military said it wanted use of the bayonet, so they used a 14.5" barrel. They said it'd be cool to mount an M203, so came the notched barrel. They didn't like the civvie handguards so made the M4 type.
I'm not for sure when exactly the A2 got replaced with a flattop, before or after the actual adoption of the "M4". All I know is that the current military M4 does utilize a flattop. There are M4 variants, so that might be what we are discussing.
I wish I could say for sure right now but I am out of town right now and can't look at the TM, but the M4 variants are listed in the 23&P. It should say which if any is an A2 and which is a flattop. I am thinking all M4's are flattops and the variants only deal with select fire capabilities, but I've been wrong before.
Someone post if they have the TM, otherwise I will post Tuesday when I'm back home and can check.