The basic K mount from 1975 is the same whether from Pentax or Ricoh. Once Pentax and Ricoh started adding extra electrical contacts on the lens mount the problems started. Just trying the combinations out can get a lens stuck on the camera body. I have no problem using plain K mount lenses on my Pentax K-x (I know, digital) but I would not chance trying a later Ricoh lens with the electrical contacts.
And now Ricoh is the owner of Pentax Imaging Company. They should fit shouldn't they?