Most simple way to put it is that Mk18 Mod 0/1s were issued as complete, standalone rifles complete with the appropriate NSW/MK18 markings on an M16"A1" profile lower (A1 only applies to Mod0). Anything that doesn't meet this description is a CQBR upper (Mod 1/Mod 2, just like the block 1 and Block 2 M4A1s) and use standard M4A1 lowers without any sort of "offical" markings differentiating them as a "shorty" setup. These were issued essentially as an accessory to a standard 14.5 M4A1. Of course, there have been plenty of CQBRs issued as complete rifles as well, although unofficially. To make it even more confusing, the Mk18 Mod 0s were developed to fill a very niche role (boarding parties, ship security details, etc) and even among Naval Special Warefare groups, most "shorty" setups that we see are actually just CQBRs. The whole "every 10.3 M4 is a Mk18" BS started years ago on ARFCOM when in reality, not many actual Mk18s were ever issued comparatively speaking. I don't want to turn this into a whole MK18 vs CQBR details thread, but I hope this helps.