I have wondered the same.
ALMOST all pen kit parts are plated brass ( exception that I know about is :- some stainless steel kits were available from one or two specialty vendors a few years ago, but don't seem to be advertised much anymore )
I don't know much about platings, but I would suspect that "Antique Brass" would be a plating.
Unplated brass, though attractive when first machined and polished, does not remain attractive for very long due to oxidation in the atmosphere.
Brass needs a lacquer or durable plating.