I found Nikon DK-17M fits into CONTAX ST eyepiece by accident, though
the finder image has slight pin-cushion distortion but quite usable. So for
CONTAX AX too, but it's hard to see the corners especially eyeglass users.
Any diopter correction lens (DK-17C) for high end Nikon cameras today
works luckily on the late CONTAX cameras such as RTS/II/III and AX too.
(btw, camera's date has been working to 2014-Oct without an error, but
I took the battery out since the camera was bought 1999 as new, $700!
The date is printed on the film between frames, which is a nice feature.)