My understanding is that in the 1860s at least these parts were either forged or cast, then shaped and finished by hand, then engraved. As Mr Barclay states above, engravers were plentiful, and hand work was relatively cheap. Even the most basic ‘trade’ engraving at the time was nice by modern standards, and better engraving could enhance the appeal of an otherwise standard offering. I simply marvel at the range of hammer styles. While hammerless guns are a technological improvement, I prefer the look and tactile pleasure of exposed hammers.