The gear ratio for the Main drive Gear and Magneto gears is two and a quarter. The contact breakers give 2 sparks per revolution and run at two and a quarter times the
engine speed, each giving nine sparks every two revolutions of the engine.
The firing order therefore is 1, 3, 5, 7, 9, 2, 4, 6, 8.
Looking from the rear, the right hand Magneto/contact breaker supplies the front set of spark plugs and the left hand Magneto the rear spark plugs.
See Magnetos simply explained for additional details.