The ULMRB-23 makes it possible to use Mamiya RB & RZ Lenses in combination with the Ultima 35.
Mamiya RZ/RB lenses can be set manually to the desired aperture setting.
The use of a Mamiya lens relates to your sensor size as the same focal length in 35mm format, so f.i. a 50mm Mamiya lens will have the same perspective on a 1Ds as a 50mm Canon lens;
for the smaller sensors the maginification factor will apply of course.