On the Tessar type Mamiya Press lenses, it is quite easy - these are composed of screw in cells on a shutter mounted with a retaining ring to the barrel, just like usual on folders or large format lens boards, so there are no tiny screws between you and the shutter. The 50mm wide angles are not as easy, thanks to an integral shutter. I don't have the 65mm, but from online images it seems as if at least the early (silver) version might follow the easy pattern.
Sevo