I purchased this 539 LNIB a few years back. It was pristine.

Since then I’ve carried it in a Galco leather holster. Not an everyday carry, but occasionally. I am very fastidious with wiping down my guns, and always making sure a coating of Ballistol is protecting them.

I noticed that on the slide, there appears to be either wear, staining, or rust. I don’t know which it is. Any thoughts of what the marks represent?

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I would love to help but I probably won't be much help except to say...

S&W's bluing on guns of this era was maybe some of the worst they've ever shipped. (my opinion) On top of that, I have never met any traditional bluing that didn't show wear in short order when used with a holster... especially a leather holster.

Not knowing a lot on this subject, I have always held this axiom near and dear to me: if I am going to carry a handgun, any handgun, in any holster, I am -EXPECTING- the finish, absolutely no matter what the finish is, to show evidence of holster use.

Of all the handguns that I have put wear on over decades, the one that has cared the least? Circa 2008, MML-prefix Glock 29. The "Tenifer" finish they used in that period of time is unbelievably tough as nails. They don't use it anymore, I believe that the manufacturing process wasn't "environmentally friendly" and the finish that Glock uses now is junk compared to what they used on my circa-2008 pistol.

Wen I was assembling these in the 80's there was a guy on my bench that caused the bluing to sort of "wear off" on the same spot on the slide when he worked the action when building the gun. Something about his hand sweat reacting to the bluing. It just came on suddenly, probably something changed in the process?? Just curious, do you grab the gun there very often?

Looks like holster wear to me , I would go easy on the Ballistol and use silicone cloth and see how it does . A mist of Rem oil on a microfiber cloth works really well .

Thanks for the input. I have read here and there that the finish on the early 80's guns was not as nice as the earlier decades.

I never touch the blued gun unless needed, and if I do, I wipe it down with a treated cloth / paper towel.

We have a old 38special Police Positive revolver in the family that has been stored in its original holster for around 80 years now. There is definitely leather wear spots on this gun just from being involved with the leather holster altho not used or worn since the early 30's, something in the leather does it to the sides of the gun. The gun looks like New except for these weird spots.

Possibly to much external oil on the gun attracting contaminants in the holster?

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Some collect art; I shoot it!

The gun is wiped down about once a month some Very good leather treatments as well but some holsters will leach material from the metal just by being in contact with it over long periods of time.