Awww, you all are trying to goad me into posting the review of xr-2 and the set of lenses...
All I can say is that I believe both Pentax and Ricoh validated what Modern wrote: the first series of Rikenon XRs are the same optics as SMC K series Pentax lenses, are manufactured by Pentax, and have less surfaces multi coated than the equivalent Pentax lens.
Remember also that Pentax had just discontinued the SMC K in favor of SMC-M, and probably had the tooling and components left over.
Who made all successive Rikenons is another matter, I doubt it was Pentax for very long.