I'll wager that red line probably gave him a clue : ) It's probably to enable alignment with the rotor gear.
Something that I've wondered about with the magnetos I've looked at is how it appears that the points are closed only for a short period, which "seems" counter to the way coil ignition works, where the points are normally closed and open only to collapse the primary circuit and induce the spark. I would have thought the magneto would have worked the same way but if so, it seems strange that the primary circuit would have such a short time to build before the points re-open. I obviously don't understand it so if someone has a simple explanation, that would be great. The cam profile in the picture above seems to have a much longer "dwell" (don't know if we can use that term with respect to a magneto) than the one on my D2