Hi Alexis,
if you go shopping for your RTS II you have to pay attention only to 2 things:
1. The lens mount
2. The colour of the smallest aperture on the lens
The RTS II has a C/Y mount, so all lenses with this mount will fit. This can be Zeiss lenses, Yashica lenses or third party lenses (i.e. Sigma etc.)
In the 1984 (after the RTS II), Contax added Program Mode and Tv to the new camera models. From this time on, the new manufactured lenses got a mechanical modification to be able to be used in P and Tv. To check whether a lens can use these 2 modes, you simply look at the smallest aperture. If it is coloured in green, then it is possible, if it is cloured in white, only Manual and Av is possible with that lens.
The RTS II has only M and AV. No P or Tv mode. So for this camera it does not matter. But the second hand price differs, so it is good to know the facts. Also if you are planning to upgrade later on to anotzher Contax camera with C/Y mount (digital or analog), it would be better to buy only the green market lenses.
Dirk