There is an explanation of paint masters in the auctions information on each one of Bill's pieces being auctionsed. Here is what it says about paint masters:

Description of pieces in Bill’s collection:

** Paint Masters (or Artist Originals) **

Upon completion of an original Harbour Lights sculpture, castings were made. Typically there were five castings made from each original sculpture. Two of the castings were used to design the paint scheme for the piece and the other three castings were used to make molds for production.

The two castings used to design the paint scheme were referred to as Paint Masters (or Artist Originals). These pieces typically don’t have any metalwork included (hand rails, posts, trees, etc.) The metalwork was designed once the production molds were made two or three generations later. The metalwork was not designed from the first castings because the pieces shrink slightly with each generation. If the metalwork was designed from the first casting it would not fit the production pieces. So you will find the Paint Masters (Artist Originals) lacking metalwork.

There is also information of counter samples, artist proofs, etc. I found it very information.