When we say a part is unfinished brass it means the brass is in an natural state after manufacturing.
Nothing more has been done to the brass to finish it.
The brass is dull, may have manufacturing marks such as small scratches or scrapes.
The brass could have manufacturing oils or grease. This can stain the brass in some places.
The brass can be tarnished. After manufacturing , when brass is not sealed with varnish or lacquer,
oxidation can start darkening the brass. Since parts are manufactured at different times unfinished
brass parts can be at different states of aging. This would make unfinished parts not match each other.
Many People use unfinished brass parts as the are, to give a old antique feel.
The parts continue to oxidize and eventually match each other.
Others will professionally finish the brass by polishing or brushing or staining with patina.
A Varnish or Lacquer is usually applied afterwards to seal the brass and stop the oxidizing.
A wax is sometimes used instead of lacquer for a more environmentally friendly finish.