I have an odd set of 915 heads. Usually their are two main casting numbers on a 60's Mopar head; the model, and the production date. But on this set of heads there is: 2780915 10, and on another area it is stamped 2255. As I understand it, that would mean they were built on February 25th, 1965, but 915's were only produced for 1967. Anyone have any ideas? There is also a weird looking symbol on the front exterior of the head. It looks like a filled in letter M. Any help is greatly appreciated.