1936 Buffalo Nickel With 3 Back Legs?

to CCF.

Yes, it does appear to be a stain.

Thanks for posting the new photos!

Your coin was struck very shortly after the obverse die was polished. The polishing lines are sharp on your coin. The key to spotting die rotation is the second feather / "U" of UNITED clash. The feather clash is visible on the reverse, between the buffalo's head and the left side of the "U". This is very slightly rotated from perfect alignment, and well within mint tolerances. The right rear hoof / "LI" of LIBERTY clash confirms the alignment. There are two known die pair with the obverse die heavily polished along the left side of the Indian;s neck. Yours is one of those two. Neither pair shows an anomaly next to the buffalo's right rear leg. Together, all of that confirms what you have observed, that this is likely a stain.

Your coin is a conversation piece, though, because is was struck so soon after the obverse die polishing. That tells a key part of the Buffalo nickel story, and is worthy of a 2x2 just to show that off.

Nice coin, an interesting piece to tell the story, and thank you for sharing it with us!