H3NRY: "Each TOPS init is serialized, so you will need at least 2 copies with different serials."

From memory, I think that the INIT on an original install disk is not serialised, and that you just type in the serial number on the TOPS box (orange label on the spine). Thus to use multiple versions on one network, you need one install set and multiple serial numbers. And because it's all AppleTalk, you don't need to worry that somebody else in the world is using the same serial numbers.