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, 990 & RZ 990 Compatible
RZ 1090 & MZ 1090 Compatible
EZ 590, 591, 390 & 391 Compatible
RZ drums are not compatible with EZ, but can be converted with special kit.
MZ 790 can be converted to MZ 1090 with special kit.