[FFR428]

Between the center 2 valve springs you'll see the date code. Number/letter/number sequence. First number is the year ending ex 6=66, 7=67. Next the letter is the month cast. A-M= Jan-Dec skipping the letter "I". So A for Jan, B for Feb, C for March and so on. Last number or 2 are the day of the month they were cast. By reading the date codes you can tell if they are close in date and a matching set. There are 2 types of C6-R head castings. Some have a large R on one end. It's does sounds like they might be a mis matched set. C6AE-6090-R as posted is the full casting number.