This is very strange! If they were trying to go for a vintage look, why didn't they swap out fixtures with a proper drop-lens mercury fixture? Particularly as Joe Maurath lives in the area and could rent out fixtures. FCO fixtures like this weren't around in 1967 and it seems odd that they would relamp an HPS fixture with a mercury lamp just to get white light (and I think most mercs for roadway use were probably clear bulbs back then). Yes, the mercury lamps will work in a 200w+ HPS fixture but they'll typically cycle in the 55v (150w and less) fixtures.