Speedometer Adapter

[Darthbaker]

I've looked and looked and I can't information on the adapter I need.

I have a 1971 C10, 355, with a 85ish 700R4 transmission. when I bought the truck the previous owner stated I needed a mechanical adapter for the transmission to make the factory cluster speedo to work.

I don't have a pic of the current connection on the transmission at this time but there's a plastic plug adapter on it where i appears to be electronic.

Any advice on where I can get this? A part number? anything....