British motorcycles prior to the mid 1960s would all be wired with 6V alternators; the earliest Lucas alternator would be the RM15 or RM18. These only gave of the order of 80W, so they were surpassed by the RM19 which, assuming that all was working correctly, gave 110W. The rotors and stators are not interchangeable between the two types, even if at first glance they might seem to be. The RM15/18 has a 70mm rotor diameter while the RM19 and RM21 and in fact all subsequent rotors are 74mm.