I've just purchased a Mamiya RB67 ProS camera (fulfilling a long standing dream to own and use a medium format SLR). It came with a 90 mm f/3.8 Mamiya-Sekor C lens, and two 120 film backs, one for 6x4.5 and one for 6x7 cm. As far as I can tell without having shot and processed a roll yet, everything appears to work correctly.
I found a YouTube video recently showing "how to" shoot long exposures (longer than 1 second) -- which is different from most other cameras and lenses I've used (since 1970). The "common" way is to mount the camera on a sturdy tripod (or similar steady support), set the shutter to "B", and use a locking cable release to open the shutter and hold it open for the required time, then unlock the release to close the shutter.
The RB67, however, has only "T" shutter above "1", no "B" setting. I'm used to "T" meaning the shutter will open when I press the release, and close when I press again, but in the RB67, the shutter opens and then the shutter release locks. Recocking closes the shutter, and the only other way I've found to close the shutter (based on a YouTube video) is to turn the shutter speed ring off the "T" setting. This works the same way whether I set the "mirror up" release on the shutter, and trip the shutter with it, or if I try to do everything with the release on the body.
The same video told me I'd otherwise need a dual cable release to use the "T" setting for long exposures. However, I haven't found any information (so far) about how to actually use the dual cable release, either with or without the mirror pre-fire, to shoot long exposures on "T" with the RB67. I don't yet have such a release, but my attempts to simulate using both releases in "T" have resulted in the same results the video maker had -- having to turn the speed ring off "T" or cock the body to close the lens.