I suspect that the catalogues do not list Rodrigues cancels, as the island has always been part of Mauritius. Unlike the Seychelles, B64 is a "forerunner"
Yepp, you are probably right. I'm a forerunner kind-of-guy, so it was my hunt for B64 that made me aware of the B65 in the first place. And as you say, Rodrigues has never been neither an independent state or postal entity. Tough Rodrigues almost started issuing separate stamps, as per info on the link provided by GeoffHa.
I wonder how come they sell for such premiums if there is no catalog - or reference of scarcity. Probably the scarcity of each classical Mauritius stamp with B65 are described in some kind of super-nerd literature - somewhere - I'm just not knowledgeable to find out where. As has been pointed out by others concerning the lack of sharing information among stamp collectors - maybe someone knew but would not share, and brought the wisdom with him/her to the grave
Frankly I'm not too concerned about the value as such - but it would be nice to have an idea of scarcity.