273 casting number question

Hello: I am in the market for a date correct 1966 273 motor. I have found a couple early casting date motors, August and September 1965 that have heads that have casting numbers 2465315. That number is for a 1965 motor. Did Chrysler put 1965 casting code heads on some early 1966 motors? I thought that the 1965 heads were not interchangeable with 1966 motors. I would appreciate any help on this. Thank you. Bob