I'll stand to be corrected on this, but I don't believe the factory really had a preference as to whether or not the stamped serial number needed to be visible. There was a bit of latitude as to where the stamped number was placed, but there was more than likely some sort of callout in the graphics books as to where the labels were put. The stamped number was likely more for in-plant use during assembly, and for law enforcement if the car was stolen, but for the "normal" car purchaser, I don't believe it mattered whether the number could be seen or not.
Also I don't believe there were inspection stamps on single pot master cylinders in 1965. I think that began much later.
