The mk 1 had a proprietary single stack 5 round magazine. The mk 2 went to standard aics mags, and the trigger could be dropped free with two screws like an AW(once the skins are removed). The mk 3 retained the mk 2 features, but used the AW bolt with the 3 position safety. The mk1/2 had a different bolt shroud with two position safeties.