Looking at an enlargement of the lower cowl photo (which is slightly better resolution than the one I had on file), I'm thinking that they painted alignment marks to either side, but also the slot of the fastener, making it look like a single line. I doubt the shadow of the slot would look that dark.
I thought there might be a thread on ww2aircraft.net which just includes photos and other information on the MB5. I did a search but didn't see one. Before I start one, can anybody point me in the right direction?