Dave,
When I I bought my truck 2 years ago it still had the original paint. I wondered the same thing and searched the internet and I have seen old photos of burbs with the same stripe. I'm 99% sure that they came that way. Maybe as a trim option or something?
I just wanted to put it back the way it came (as far as the paint goes,,,) . All of the paint under the hood is original. I thought the crayon marks on the firewall were cool so i left it alone. Just a mild cleaning. Oh !! the white stripe is only on the doors. It dosent extend around the tail lights... I have some original paint pics I'll try to post tonight. I have to get to work. See Ya ! Steve