The post ‘74 Contax SLR cameras are Yashicas, it’s not just a shared lens mount. The cameras were advertised as such too, the original RTS baseplate has Yashica deeply stamped on it!
There’s al lot more too; The FR series are styled as baby RTS bodies and the Real time Winder and Yashica FR winder are largely interchangeable (although the RTW has additional features), as are the 139 and FX winders. Then 139 & FX-D share quite a bit under the hood, as the 159MM & FX-103. The TTL flashes are interchangeable and this is covered in brochures and manuals as are other shared accessories like the electronic cable releases. And so on.
Also, IMO As Kyocera was not a camera maker prior to the Yashica purchase any Kyocera labelled camera likely came from a division rooted in Yashica and should, in my view, also be included.
On the other hand, the Contax rangefinders and earlier German made Zeiss cameras have no relationship at all to Yashica and should not really be discussed here. There’s probably a bigger case for discussing Pentax lenses derived from their discussions with Zeiss (eg the 28/2) as they appeared in C/Y Mount.
I agree completely. It's just gummy to decide where to cut the NUMEROUS strings. In many ways "Contax" was Yashica. In many ways "Tomioka" was Yashica. In many ways "Kyocera" was Yashica. But in many ways, each was not.