I have looked at proof marks on a lot of original finish Winchesters (many were M70s made from 1936 to 1963) and only a small fraction had bright metal at the bottom of the stamping. Over the years, the bright metal that was there originally usually faded–perhaps lightly rusted. I rely more on raised metal around the proof mark (which gets buffed off with careless rebluing) and other clues like polishing marks and color. Pre-war rifles with rust blued barrels are hard to fake perfectly when reblued. Of course, on lever actions, always check to ensure that the radiused edges are razor sharp as they were when they left the factory. Buffing often rounds them slightly.

All proofmarks are not alike and you can be fooled both ways–accepting a re-blue as original and rejecting an original as a re-blue. Problems can arise when a rifle is "mint." Sometimes that results in buyers being overly suspicous and they may pass on an original rifle.