I am nearly certain they accomplished it by burnishing, which closed the pores and scratches from manufacturing. I have a 200 year old London double flint shotgun whose internals still shine like a newborn baby’s butt. And no rust! But how?

I have successfully burnished brass to that degree of finish but brass will flow with enough pressure. I just cannot imagine hand labor exerting enough force to flow iron or steel.

When an old man dies a library burns to the ground. (Old African proverb)