I've got a slightly modified (mostly cosmetic) Springer 9MM Loaded that the slide won't always stay open after the last shot. I've had the gun for several years now and have put somewhere between 500 and 750 rounds through it. I tried a slide lock from one of my Kimber 9MMs, and the problem goes away.
My question - If I can't fix the problem with the current slide lock, do I need to order a specific slide lock for a 1911 9MM or will any quality lock work.
9mm slide stops use a longer lug that extends into/through the frame cut than a 45 slide stop...
Have you tried different magazines?
I ask this because the magazine follower pushes up on the slide stop lug to allow the lug to catch the slide notch... .if the mag follower is too low then it won't fully lift the slide stop as shown in your second picture...
Sounds like the slide stop is at fault since another 9mm stop solves the issue. When you order a new stop, make sure it is for 9mm.
I've got 11 mags and they work just fine in my other three 9MM Kimbers, so it sounds like it's time to start looking for another slide stop for the Springer.