It would be a story from all of the above comments rolled into a conversation about why its there... Since there are varying opinions as to why its marked on the WS, there are many references one could reference from this post alone.
One could tell that the marking could be found on the right or left side of the center of the windshield... One could tell someone it seams to denotes the center of the WS, (which seems to be the most logical to me), or its a QC trunk mark for water leaks testing.
Basically, the same story the guy told me, in that, many original photos of survivor cars shows this marking on the WS, but not really sure what it really means.
Unless, I missed the real reason its there...