The Dyna S uses a much longer dwell than the factory ignition. This can interfere with electronic-tach operation. In that case you would need an adapter circuit between the tach and the 1-4 ignition coil.
it occurred to me that the black wire from 1 and four is basically marooned and if I were to jump between from the new 1&4 lead to the old 1&4 which I nipped back and taped up under the tank that the tachometer might get the feed it’s looking for?
currently the Rev counter just reads zero.
there must be a splice off 1&4 that feeds the tacho right?
Yes, correct on both questions.
That black wire previously went from 1-4 coil to the original igniter. The tach signal wire connected to that black wire.
If you connect that black wire back up to the coil in addition to keeping the DynaS wire on it, you will then be feeding a signal to the tach.
Try it. The tach should at least bounce, and may even show RPM, especially in the lower range.