I think the handle on the S6 was first made for the Busse Taliwhacker.
It was used in the Swamp Rat Battle Rat, Camp Tramp and Ratchet.
So far there are only 4 or 5 variations of the Resiprene-C handle. The Busse Basics had one or two versions. Swamp Rat had the mudder, the large handle like the S6 and a smaller handle which looks fairly similar.
From what I have heard, the tooling for a Resiprene-C handle is fairly expensive. I don't think we will see any new handles.
As for the shape of the grip, I think it was inspired by the Busse Straight Handle from the Steel Heart 1.