Did a 65 Merc 427 even have those valve covers? Or did it have the Ford chrome pentroofs? That would have left extra breather caps unless they blocked the intake one.
I have to believe they were an accessory. I've had several sets that appear to be factory chrome and others that were painted that I took to the chrome shop. I've also had 58-59 baldy valve covers that appear to have been chromed back then.