I'm not sure if the retaining rings are original. They seem like it, though, and I used to have another 18cm lens that acted the same way. It would not fit my C33, but it fit a C220 fine.
What/when was the first model that had auto shutter cocking? Was this feature there on the C3, or did it start with the C33 (or maybe even the plain Mamiyaflex)? And what was the first year that the 18cm lens came out? What I am thinking is that perhaps my lens is very early, maybe made when auto shutter cocking was not yet available on any model. Since the auto shutter cocking mechanism makes the mounting hole in the body smaller, perhaps Mamiya started using smaller retaining rings when the first self-cocking model was introduced. If this is the case, then knowing whether or not this can be indicated by the use of 180mm as opposed to 18cm would also be a good thing to know. It would also be good to know if this is the case with any other early lenses.