Tach drive distributor, what is the deal??

I have a couple of them, just put a NOS one on my 416 stroker engine that is going in my 69 Dart 340 and I have one for my old gasser project.I will look to se if I have another lying around. I think they are all vintage.I lucked out and found a guy that had a bunch of NOS 340 stuff and the distributor was part of it.