Also, if any portion of a scope mount base is even slightly ill-fitting in comparison the the surface it's screwed down/torqued down tight on, there's usually going to be SOME indication a scope base was once there - probably a line from the edge of the base, through the bluing, that shows bright.
It's a good idea to wrap a piece of emery paper around the receiver top (abrasive side "up") & rub the prospective scope base(s) back/forth atop it, to rub off/even any possible irregularities on the bottom of the base(s).
FWIW, I've also found that what looks like corrosion (on commercial rifles) is usually dried thread locker - which can be easily scraped off with a wooden chisel cut from a popsicle stick, leaving no trace.
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