I would say late sixties/early seventies. In the early days there were so many types of driveshaft (refer to the Haynes manual to see the array). I cannot pin them down to a specific year. Best to visually match them up with your existing driveshafts. Somebody on the forum might be able to be more specific. Best Wishes
Greetings from Zimbabwe. I am in the middle of the restoration of what I believe is a 1972 845cc R4. The drive shafts are knackered. Are these still available? Where are they?