There are several Risograph Z-type drums, most of which are not universally compatible.
Use the following charts to determine compatibility with your machine:

North American Models

RZ 590 & 390 Compatible

MZ 790 & RZ/MZ 990 Compatible but Drum Code Entry may differ
         MZ 790 Drum Code Entry (Test mode #1210): 69
790 can be converted to MZ 1090 with special kit