I noticed the 'pockets' (is that what it's called?) are different on these 2 A2's. The upper gray one is a GC serial and the lower black one is an LGC serial. Also notice the difference in BCG's. Why the difference?
Top one is a semi-auto only upper receiver, with a semi-auto cut bolt carrier. Bottom one is a full-auto cut upper receiver, with a Colt semi bolt carrier cut for a lower with a denial block.