Sanj seems to indicate the electronc speedo's started with the S4S, not the S4, and that the S4S and later cars went to the Caerbont unit instead of the earlier mechanical VDO.
I have not pulled mine yet, I need a day or two away from the project. After coming back from a test run the other night, I was quite thrilled to find the roof was no longer making any noise, and the car was perfectly quiet. Then the next day I discoverd the passenger window part way down, crooked, and partly out of the track. Yet another unexpected task on the list. I know, it is a lotus.
Anyway, my car was built in Sept94, registered as a 95, and it certainly does have a vdo speedo in it. Does the setting at 30mph deal when the car is running, and jumps up intermittently when giving it some throttle. That seems to match the vdo mechanical magnet problem.