Thanks Clive - given your info and the serial number it is safe to say that the magneto is a 1912 Bosch Arc Light.
I don't have access to the article that you mention, so one final question.
What years was the Bosch Arc Light fitted?
Looking at illustrations, I can now see the horizontal cable on the 1912 models, but by 1914 the magneto seems to have changed to the Bosch ZA2 like this one (that has a 1914 Bosch serial number 16..... - the rest is mashed by injudicious use of a spanner on the mounting nut). Presumably the change from Arc Light to ZA2 was sometime around 1913?
Cheers
Leon