Another bit of Synchro/Syncro info
 

I have recently been experimenting with 7 speed Syncro shifters and a 7 speed Regina Synchro (Extra 90) freewheel. One of the best pieces of info came from Bianchigirll who said the rear wheel axle should line up with the dropout;this really settled down the shifting. I have experimented with the Athena (blue insert) and Chorus A (black) and B (green). All worked well with the 7 speed Regina Synchro freewheel. I did however notice that the cogs seemed to have the same gap as an Everest Nova 6 speed I have. I measured the cog spacing and to my surprise, the cogs spacing for the 7 speed Regina Synchro freewheel are 5.35mm c-c, which means rather than having the standard 7 speed spacing of 5mm, the cogs have 6 speed spacing. Since the 7 speed Regina Synchro was designed by Campagnolo to shift the best with their Syncro shifters it would seem that 5.35mm is the optimal spacing for the 7 speed Syncro rather than 5mm; granted the accumulated tolerance on a 6/7 speed freewheel would only be about 1.5- 1.8 mm, but the 5.35mm spacing is still worth noting.. I installed the 6 speed Everest Nova freewheel, adjust the stop for 5 clicks (6 speed) and it works as well as as the 7 speed Regina Synchro, which is actually quite acceptable.

I think you may have uncovered both the charm and the curse of Synchro. Because it was designed to more or less retrofit a variety of derailleurs and FWs if one is willing to play with it I think you could make it work with most anything.

I wanted to tinker with it last summer but I could not get the shifters to fit the frame I wanted them on. I thought Campi or someone had made a shifter boss milling tool but I could never find a reference to one anywhere.

Good info. I have a green 7 speed insert I want to try with shimano freewheels.