New To Me K9 Questions

[iWander]

I just picked up a new to me AR28XX K9 I won on GB. It appears it was well maintained other than a VERY wet striker and striker channel. I've looked around and don't see definitive answers to the following...

1. The slide rattles at the rear when on the frame. The slide and frame rails appear to be straight and damage free. I'm recovering from foot surgery so I can't shoot it for about a month. Has anyone else had this issue? Should I be concerned? The only other gun I've seen that rattles like this one was a shot out Beretta M9. My only other metal framed Kahr was a Lady 9 twenty years ago that was very tight. None of my P series guns have ever rattled.

2. What does the 6 or 9 mean inside the frame?

What does the C and two dimples mean in the slide?

Thanks!https://uploads.tapatalk-cdn.com/20230129/0faab26d950b82bb06b82af972e44c0d.jpghttps://uploads.tapatalk-cdn.com/20230129/37ce13c2d73fd0f91df9cca57580e26d.jpghttps://uploads.tapatalk-cdn.com/20230129/843430126cf3aa78144f2d78cf7ecca3.jpg

[ripley16]

The dimple means the sight dovetail cuts are the "new style", as opposed to earlier models. Can't say what the C or the other mark means.

[iWander]

The dimple means the sight dovetail cuts are the "new style", as opposed to earlier models. Can't say what the C or the other mark means.Thanks Ripley

[FreeMe]

Ripley is correct, and I also don't know about the C.
Don't worry about the rattle. It's actually not uncommon in autos, and not generally important. What matters is the barrel's fit to the slide. As long as that is close, you're pistol will be as accurate as it can be in your hands (maybe not from a Ransom rest though). The slide is where the sights rest, and the barrel rides in the slide. Loose rails (within reason) just make your pistol more reliable.