SIP Speedo Version 1

I have bought a 1978 Primavera with the first version SIP speedometer fitted. Everything works except the speed. I have checked the drive at the wheel and it all works but when I insert the drive into the back of the speedometer it will not turn- it is totally jammed. I bought a digital speedo cable but this version speedo does not have a socket to plug this into, just sockets for the black box, the temperature sensor and the option button. 

Is there any way to fit the digital speed sensor to this version?

Is it possible that the inner metal speedo drive is too thick for the unit? What size thickness should the inner cable be?

Any other suggestions?