Leica R mount would have been a Leitz lens, not Zeiss. Contax could have had a comparable Zeiss lens.
Actually, Zeiss made the Leica R 15mm 3.5 for Leica.
Though my question is now moot, since I now remember that the
Leica R 15mm 2.8 was made by Schneider, not Zeiss, so the new Zeiss lens really is new.