Those may be the original "born-with" heads. The casting dates on engine components were very rarely a "to the day" match. Think about it. Mass production, building as many cars as possible day in and day out. No one knew at the time these cars would one day be the collectors items they are now. GM was in the business of building and selling lots of cars - period. As such they had huge piles of engine parts: blocks, heads, etc. that were being used to build engines constantly. When it was time to build an engine, they just grabbed the next parts off the pile and put them together. G117 (July 11, 1967) and F277 (June 27, 1967) are plenty close enough to each other to have been the original build. The engine foundry started building parts for the next model year "about" mid-year previous so those dates are likely to be for model year 1968. 16's (216's) were used in 1968 on 400's, block casting 9790071, engine codes YW and WZ according to Wallace Racing's site, but that site also shows 16 and 216 to be the same --- so could have been on the HO 400's (YW and WQ) also. I don't know if it's possible to 100% guarantee those are the parts that rolled out of the factory together, but it'd be very tough for any judge (if that's what you're interested in) to say that they didn't.