I think the gun maker, target shooter, self promoter's first name is Willard (Milton Farrow)
His father was William
Willard had lots of brothers, sisters and 1/2 siblings. ,,2 of the latter were named William I think.
Not an uncommon name at the time.

Try the Single Shot forums for info on him, They research the makers quite deeply and would likely have uncovered any hint of making or retailing a SxS sometime in his career.