This depends on what type of magneto your Dad used on the bike. The original was a Lucas Magdyno that had no kill switch fitted. It didn't need one as the rider used the exhaust valve lifter to stop the engine.
As you Dad used the bike for racing he may have fitted a different type of magneto and its a bit difficult to give an answer for a mystery magneto.
If the bike had a kill switch fitted it would be to meet some local racing rule. It was otherwise unnecessary, it wasn't needed even if the bike was used for racing.
Cheers, Bob
