Just use a Dremel tool with with a medium grit polishing wheel and polish off the hardened surface. When the checkering file 'bites' into the metal, you'll know you're thru the thin hardened layer.

You can also just polish off the hardened layer by hand with 400 grit paper. Use an eraser as a sanding block and it will help maintain the curved surface of the top of the hammer. Or mount the hammer in a padded vise and use a 1/2" strip of emery cloth like a shoe shine rag.

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Jim
S&WCA #819