The British Hillclimb and Sprint championships have use the beam breaker for years. The cars have to have a 'beam breaker' plate at the front and this is lined up at the start line by lights. Good enough I would have thought. As I posted earlier, if you competed without the front plate, timing was only to 0.1 seconds.
I'm guessing it came in at the beginning of the 1971 season based on these two pics, both taken at Prescott.
1970
1971