I dunno, I'm not really sure the mag catch is a high-wear item.
Besides, with a steel or Aluminum part, you can guarantee that it WILL wear out the plastic tabs on your UMP mags, and eventually that expensive UMP mag is now useless.
Whereas, if the softer plastic part wears out after a few thousand magazine changes, it's a VERY quick and simple part to swap out and replace. Not to mention cheap. A whopping 16 dollars from HKparts.
Just my opinion though!