Thanks everyone for the reply's. I've heard the old "F" casting lore. The last set ("M" casting 906's) I used on the car finally broke. They cracked under the center two exhaust ports. I have a good collection of head castings on the shelf. I'm going to use a set of "F" casting 906's. This is the nicest set I have, both heads cast with in 4 days of each other in Sept 1967. They both have no noticeable core shift and originally factory valve job with no valve recession. I have seen all the different markings on these heads, I was hoping someone who was around back then at the factory had some insight. Thanks again......Jim