Colt M4 - military vs export
 

I was looking at Colt's website and it listed the military models as the 920 (M4) and 921 (M4A1), while the export versions were the Model 977 (A1) and 979 (M4). How do the export versions differ from the US military models?

I believe that tere are basically no differences except the markings. The original weapons are marked "Property of US Government" but the commercial/export guns omit this. I think there are some instances where they have even given some guns various export/commercial model numbers to specify different contracts for exactly the same weapon.

I imagine that the reason the numbers are switched around in this case (auto having a lower model than burst for the export guns) is that I believe that originally the 977 was offered as a commercial variant first as auto guns tend to be more desirable for foreign contracts.

There are some instances with other Colt guns where the commercial variants are actually potentially different, such as the many commercial/export variants of the M16A2. These could have a lot of changes such as different barrel profile, A1 or A1E1 upper, full auto FCG, stuff like that.

Different receiver markings certainly make sense. I've always assumed "Property of US Government" was the exception rather than the norm, though. I don't think Colt marks their civvie ARs in any such way.

They don't but they are different model numbers, eg. LE6920. There is no civilian Model 920, only the Military version which has the US Gov markings, and the commercial/export version which is the model 979.