It's easy to demonstrate what you're describing at home. If you check a piece of steel with a magnetic when it's heating up, there will be a point when it suddenly becomes non magnetic. If you remove the heat, the piece will stay non magnetic while it noticeably cools and will slowly regain magnetism. It'll still be in decalescence much cooler than it took to put it there.
The interesting part of Doc Gaddy's comment was that he didn't catch the temp of the steel coming down, he brought it down to below 'critical', and then held it for an hour. The other interesting tidbit is he said that case hardened original receivers tested to have pearlite in the thin case layer, not martensite which would be the form it would be in if it were hardened steel.
I think file 'skating' and 'glass hard' is a bit subjective. When I ever do it, I'll only try it where there's no chance of showing. Just because the file doesn't 'bite' into the steel doesn't mean the finish won't get scratched up. I think case coloring is sort of playing games with poor heat treating practices in exchange for appearance and fit. I think that's Doc Gaddy's point. Only thoughts is all.
