The shutter design in the Mamiya TLR lenses I've handled appear slightly prone to binding due to uneven pressure on the face-plate, or the retaining ring being over-tightened/missing its set screw (and allowing it to tighten due to vibration during shipping). Pressing on the top of the shutter's face plate may be all that is actually required to free things up. Backing off the retaining ring is a bit more involved, but not a big deal if you aren't bothered by the risk of 'sudden and expected complex puzzles'.
You may still want to send it out for servicing even if you can resolve the binding issue, as there is lots of room for gunk in the shutters, but to keep the binding risk in mind when it comes back. Entirely possible to have one jam in the field, and the fix be simply pressing on the top of the face-plate to the shutter's housing.
