Steve,
Time to 'pick nits."

When you say "definitive maker" - more precision is necessary. William Powell & Son
made their pinfire 'lifter guns' for about 50 'makers' - some of whom are on your list.
For instance - is William Golden the "definitive maker" of a gun made by Powell but
marked "Golden."

Technical point - you have two entries for Rigby. Same company but different dates of manufacture.