Don't know as I don't have any examples from your period. I can offer that later cars used a red "P". There was one later example that showed a black "P" so its possible that both colors were used and possibly the colors indicated different inspectors during that production period
You can always just make a batch yourself. Of course they will likely be more correct/like original but will only last a short time if exposed to water or humidity. Like the originals
Did find an example in my files for a 66 Dearborn car from your cars period but it was of the trunk and the sticker that was typically placed there.