I would rather say that they are completely different, the Imagon being a simple cemented doublet with all glass behind the shutter (it has no front lens), while Mamiya's SF is a triplet-derived lens with undercorrected spherical aberration.
Seall, I don't know if you have understood that in order to use the Imagon by means of the Zork Tubus on the RB67 you also need a shutter, as the Imagon comes without any shutter, so you have to "borrow one" from a Mamiya 180mm lens by unscrewing the front barrel.