It was Campagnolo's wish to make a system that would be compatible with their older derailers and other products, to provide an easy upgrade path for their existing customers. This was a laudable goal, but turned out to be impractical.
The Synchro (and Synchro 2) down tube shift levers had replaceable, color coded «indexing discs» with detent notches at different spacings. There were quite a few different indexing discs available for different freewheel models and spacings.
The problem that sank Synchro was cable travel. Campagnolo derailers had always uses a very short cable travel from gear to gear. The shorter travel came with higher tension, and these two factors exacerbated issues of cable friction and stretch. As Shimano also discovered, a key to good indexing
Currently, Campagnolo's shifters and derailers use an even longer cable travel than Shimano does.
Synchro derailers and shift levers are incompatible with anything else, and the Synchro system worked so poorly that it is not a good idea to try to set the system up on a bike you intend to actually ride.