A buddy was wondering why his mags were giving him trouble latching in. I started to examine it, thinking for all the world it was a polymer piece, ala Glock. Very sluggish in return, so I put on my old-man 2.5x reading lenses. Turns out that brown stuff was NOT caked-on brown AZ desert dust, it was rust, LOL.

Removing the latch was intuitive, just careful manipulation of the tip of the spring with a couple of jeweler's drivers. Rust had built-up on both ends about equally, but had not yet migrated across the whole part. I scrubbed it all off, and that which was stuck onto the polymer of the frame, also. Once lubed, and installed, no more problems.

The problem was, why couldn't I get it assembled? I could not get the slide stop to push up into place to lock the slide back open. A quick net search showed the answer. I didn't have the head of the recoil assembly pushed down far enough against the base of the chamber. I had it centered from left-to-right, but not seated deeply enough. When viewed from the side, the guide rod should not be parallel with the slide rail, it should be angled inward with a bias toward the chamber. 'Nuff said.

Check your gear, folks.

It's a good reminder from you, and shows that even out in Arizona, the mag release can rust.

I saw a similar story here a couple of years ago, and checked my Shield because of it. The mag release was almost seized with rust.
I thought it was my IWB carry in humid SC, but your dry locale proves it can happen anywhere.