The Zeiss and Leica M grips are actually shaped very similarly, with the threaded fastener in about the same place. I just tried putting the grip from my M2 on the Zeiss Ikon and... No go! The main issue is that while the Leica has a uniformly rounded end, the Zeiss body is squared off on the right front edge. That prevents the Leica grip from fitting closely enough to fasten. Secondarily, the Zeiss body has round extensions on the bottom surface, one for the rewind and one for the grip fastener socket that call for matching rounded recesses in the Zeiss grip, which the Leica grip doesn't have. I think one could use a Dremel tool to grind away some of the Leica grip for that squared corner on the Zeiss but it still wouldn't fit all the way down because of the rounded bottom parts on the Zeiss body. Close, but no cigar!