All information was taken from http://rgbcharmer.kt.fc2.com/rgb-kankyou-x68000.htm

X68000 monitors support all analog 15khz, 24khz, 31khz RGB modes. The only exceptions are CZ-603D, CZ-606D, CZ-608D which do not support the 24khz mode. The monitors come in three colors GY - gray, BK - black, TN - titanium. You will need to construct a sync separation circuit if you want to use it for other systems besides the X68000. You can use LM1881N for that purpose. If done right, the IC will will fit in the connector of the cable. It is also a good idea to add a stereo jack socket for external speakers and female RGB socket.

Sharp X68000 analog R G B monitor list