It may be soft enough to cut w/a file as it is. Some are,, even though the original finish was a case hardening. The 'case' was non-existant on some and only colors existed.
A quick check w/a file will tell you. It'll either bite in or slide off like a piece of ice.
Even if it still original case to it and I felt like making a project out of it (the coin/gray finish being the goal as you stated),
I'd go ahead and take the fences/recoil shields down cutting right through the case.
You can hand polish thru it, it's really not that deep and 100grit cuts quick. Machine polishing is even easier of course. A belt grinder is a blessing if you are confident on one. It can ruin stuff in a blink of an eye if not.
Just do the recoil shields/fences of the frame in cutting thru any case hardening. It's the fit of the bbls to the frame/fences that is the concern.
Nothing else about the frame really needs to be altered (unless you want to go there).
Leave the sides and bottom of the action w/the case in tact but you can still polish the surfaces up and match up the look to the altered fences.
No annealing & re-heat treating necessary.
It's actually not all that much work as far as the frame is concerned.
Polish out the rest of the bbls and rust blue.
I'd probably polish and rust blue the top lever, trigger guard and forend iron to match the bbls and contrast the grayed frame.
Just some ideas.
I've done projects on guns certainly not worth the time and effort.
But sometimes you just feel like doing them anyway.
I just finished up a Meriden single bbl of all things.
Have fun with your project.
