I have a list with Jawa 2 valve serial numbers by year from 1966 till 1978, but how can you find the production year of Jawa and ESO's made before 1968. For some time the year number was given as a part of the engine number. When was this practice stopped (maybe 1968?) Does anyone know the engine number or year the side drive cases changed from 6 webs to 8 webs. I am specialy interested in all the small different details from old ESO engine, like rocker valve cover, cooling fins on the barrel, etc.