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 does
CONTAX AX too, but it's hard to see the corners especially eyeglass warers.
Any diopter correction lens (DK-17C) for high end Nikon cameras today
works on the late CONTAX cameras such as RTS, II, and III too luckily.