I'm sitting here with a NON-MA compliant MP40 and a MA compliant MP9. I can tell you without a doubt that the replacement of the sear assembly is NOT the only difference in the MA version. The trigger return spring on the MA version is MUCH heavier! I'd guess a pound or two. No kidding! I'm kinda bummed. Is removing the return spring difficult? I'd like to swap and try the "normal" spring on my MP9.