Hi Brant,
This has been a long time topic for me,(1974 to be precise)and very few people work on getting this detail even close to correct.After seeing my 65 Dearborn Fastback with a 03J build date with brushed on black for the pinch weld along the rocker,I noticed later built cars appear to be sprayed on,and the overspray pattern makes me suspect it was automated,and done just prior to the installation of the front fenders.It is one of many very small details that people overlook when a car is repainted,and one of the first give away clues that it is no longer original paint.Ford still does this on certain cars/trucks so it has not changed the original intent of drawing your attention away from this seam (or attempting to hide it from view)
I hope this helps give you some idea of a time frame..I would say any car produced within the March-September 1964 window that are Dearborn produced should be brushed on.Has anyone seen a later build with this detail?
Regards R.R.