I have two commodore 64s, each with a different monitor plug. The cable that came with my 1702 monitor only works with one. I guess thats similar to the problem you are having. I have no idea why they made different ones. Maybe different countries? That was my guess. I'm thinking the external RF ...

My first VIC-20 (bought in Oct '83) must have had a built in RF modulator. If I remember it had a black cable sort of like an old stereo speaker cable that plugged into the back of the computer and the other end plugged into one of those selector boxes (Antenna/computer). A few years ago I aquired ...