I'd hardly put the Walker-Hepburn in a "class all their own", as rare as they are, they are simply a Hepburn #3 with the Louis N Walker modification. A modification that (as noted) several other gunsmiths did also.
Without the Hepburn to begin with, each modified Hepburn would be credited to the smith who modified it. A major injustice to LL Hepburn, who came up with this noteworthy design.
Just my opinion.