The marks on the rotors came about the same time they put inspection windows on the primary covers, I think this was c1967. As all rotors with timing marks are made equally The factories had to either index the position on the primary case to suit alignment of the timing mark on the rotor. Another thing worth adding is that the early cranks with 2 sludge trap plugs, do not have the same key way position on the crank as the later cranks, why bsa couldn’t have kept this the same and used a different position for the pointer is anyone’s guess, however they didn’t and if using an early crank this part needs to be made aware of.