You fit the hammer/cock to the tumbler. You try to file that tumbler shaft with a regular file, you will need a new file if the tumbler came hardened, as they are usually harder than the file. And as stated, you need all the meat on the tumbler shaft you can keep.
The hammer/cock should be a Tightfit that you have to drive on (within reason :hmm: ) :hmm: & punch out. If not, it will wear the hole in the hammer/cock extremely fast if you shoot the rifle much.
Absoutely DO NOT try to walk the hammer/cock off the tumbler shaft with a screwdriver blade back & forth. That is a recipe for disaster. You always Punch them out with a square end brass punch you make from a piece of 1/4" brass rod. The square part being just barely undersize of the square hole in the hammer/cock and about 1/2" long on that square part so it don't wedge into the hammer. I make the rod about 4" long so it is easier to hold.
Keith Lisle