Nobody actually owns a Zeiss Ikon yet; it hasn't been released to the public. Going by the published photos, I'm guessing its eyepiece is considerably larger than the others.
If the FM3A takes the same eyepieces as the FM2n, then it does share the same eyepiece size with the R-D 1. When shopping for Nikon diopters to use on an R-D 1, you've got to buy the newer ones with the non-knurled rim. The older ones with the large, knurled metal rim have the same thread size, but their outer diameter is too large to work on the R-D 1 -- there's a bulge below the eyepiece that interferes with the rim.