HP9845 printer timing belt

This talk of timing belts has prompted me to post about the one in the HP9845 internal thermal printer (one of those machines is in many bits on my bench at the moment...), Fortunately I am not looking for a replacement belt -- I say 'fortunately' because I've enver seen anything like it. There are 3 sporckets. One on the stepper motor spindle at the back. One on the platten spindle. One on the feed roller spindle. The first 2 are conventioally driven by teeth on the inside of the belt. The last goes against the outside of the belt and engages with teeth there (of the same pitch as the intenral ones). Of course that means it turns the opposite way. I've never seen a betl with external teeth in anything else (although I have seen a chain drive with sprockets engaging on both sides). I think I mentioned that the printhead in this printer can be taken apart. It consists of a coonventioal PCB to link to the cable harness, 7 ceramic hybrid circuits that contain the printhead drivers (and serial-parallel shift registers), the printhead hybrid iteslf and a load of zebra-strip like connectors to link everything together... -tony