Kevin,
I know you know what you're talking about. I quoted 455SD cuz he's the one possibly confused. [:D]
TMReich,
That wasn't an A1 upper tho it may have been mounted on an M16A1 or GAU-whatever. The change in the upper from M16 to M16A1 was the addition of the forward assist. The "field sight" forward assist brass deflector combo wasn't standardized in a military until the CF got into the M16 game. Other groups had bought rifles with the combo but had never named them.
455SD,
Besides the "4" (which can be faked) what are the other markings on the upper? Are the ramps shiny or anodized? Is there a mark on the right side of the upper forward and slightly above the ejection port?
The extended, dare I say, "M4 feed ramps", show up on quite a few uppers destined for use on carbines. The faster bolt velocity of the carbine can cause low feeds, especially with "commercial ammunition", and the M4 ramps were the fix for that. Remember, nothing (configuration-wise) is impossible where Colt is concerned.
[speeling]