Looking through all of my parts books I see that the A65 Lightning from 1966 to 1970 (pre OIF models) has the 19-2189 RSM3003/06 tachometer fitted.
I have looked at photographs of the RSM3003.06 tacho and I see that it has the ratio 3-1 printed on the dial plate.
The tachometer on my 1966 Lightning has the number RSM3003/01A on it and a ratio of 4-1.
This may explain why it never worked correctly and chewed up the end of the tacho cable on a run to Scotland a few years ago, I ended up taking what was left of the inner cable out and running the bike without the tacho.
I am going to try to get hold of the correct tacho for my bike but does anyone know what the RSM3003/01A 4-1 ration tacho is for?