The wonderful world of Google brings up this from a Triumph site
Bosch Magnetos
Year Serial
1902 14147
1903 24131
1904 42571
1905 75753
1906 123210
1907 185329
1908 251268
1909 434566
1910 646345
1911 933508
1912 1335331
1913 1567965
1914 1758030
1915 1798093
amd this
What is peculiar to Bosch and useful in dating, is that the magneto serial numbers were sequencial across all models. There was a list of these serial numbers wrt year of manufacture tha was printed in a VCCAustralia(Victoria) newsletter about 30 years ago. I moved this a couple of weeks ago, so it should not be hard to find for you. Bear in mind that this is the year of the magneto rather than the car; but usually it would be fairly close.
and this
Bosch Mag Details
This may assist in getting the right Bosch mag for the job.
Z = closed magnet. ( D = open magnet )
E = 45 m.m. spindle height. ( A = 35 m.m. spindle height)
V = vee twin ( 1 = single cyl, 2 = 180 degree twin. )
Slip ring determines whether the mag is 42/ 45/ 50 degrees.
D series mags with open magnets generally finished in 1911 although you will find some 1912 models fitted with them.
I do have a list of serial numbers for Bosch mags from 1900 to 1915 and could post them if readers think its necessary.
1915 Bosch mags started from serial number 1798093.
Examples: DA1 is an open magnet,35 m.m. spindle height, single cylinder mag and the ZEV is a closed magnet, 45m.m. spindle height, vee twin mag.