Traded for a Mk III. The previous owner wasn't the original owner, so I have no idea of total round count, but it doesn't look all that used. The person I got it from changed the hammer, sear, trigger spring and recoil spring. And either he, or someone up the ownership chain, disabled (or whatever you do to it) the magazine safety. The gun shoots great and has so far (~500 rounds from me) had zero firing malfunctions. But I have noticed one strange thing: the safety often won't engage. It happens mostly after firing a round or more, and sometimes after I just rack the slide. The slide is in battery. If I pull back on the hammer, as if thumb-cocking it, the safety will then engage. It behaves as if there's something that doesn't quite reset unless I rack the slide really hard. Ideas?