>>95877579
mk3 is mk2 with plates
mk4 is a new design from the ground up
mk6 is a new design from the ground up, but designed for better compatibility with older armor parts for maintenance
mk5 is mk4 without all the fancier engineering, but is also more compatible with older parts
Info on compatibility is contradictory, as a SoH in mk3 on Istvaan III looted mk4 pants, but IH sticking a mk4 arm on a mk3 is an incredible feat.