I have a Mamiya M645 Pro along with Mamiya's extension ring, 1, 2, and 3. I understand the basic premise of extension rings and what it does, but I am not sure if there are special considerations in using Mamiya's implementation of the device? For example, Hasselblad version require that ER's be attached to the body first, then the lens; then taken apart in exactly the reverse order. I'm particularly concerned because the lever that actuates the aperture to close to the set value upon firing the shutter can possibly in wrong position and that could require certain caution and attention when putting together the assembly. There are 3 alignment marks on the ER body... what are these for? (one is obvious) Can anyone assist?