1923 Peace Dollar (Unusual)

Give it a good long soak in acetone.

Dont use finger nail polish remover, buy a can of actual acetone at you local hardware store.
It wont hurt the coin or its value.

Where is mint mark? I dont think its glue either or epoxy, it was in plastic coin book, I tried scratching it and its not glue.

I am also in the something-on-the-coin camp. Worse, it looks to me like someone might have already tried to scrub whatever it is off with a wire brush or abrasive.

Very interesting.
At first glance, it looked like something on the surface, but ....
It looks like the light is coming from about 9:00 and the glare along the edge under the T (LIBERTY) does make it look like the area is incuse (not raised like an adhesive).

There appears to be an incuse area under the chin as well that seems to have the same dull grey toning. I'm wondering if this could be struck through some type of foil or metal.

The area under the chin would have come off long ago to have toned like the rest of the coin.
The brighter area would have come off more recently.

Can you provide an image of the reverse please?

Looks like it's been harshly cleaned to me.

here is the back and Smat it is an s