I have a Metalform mag that works perfectly in my Mk. 1. In fact, one of the reasons I prefer it to other mags is that it doesn't ride quite as high in the gun as others; I was having a problem with my magazines contacting the ejector, and loosening it, during firing. Even the factory mag had enough vertical play that it contributed to the problem. The Metalform mag fits perfectly; it locks in, and the floor plate contacts the bottom of the frame before the top of the mag hits the ejector.