My book shows that the 64-65 273 head (Casting Number 2465315, 57-64.5 CC) was the first 273 production head. In '66 the 273 came with the 2536178 casting same CCs. Then in '67 Mopar changed the casting number to 2658920 but still had the same CCs. My book doesn't specify "transition dates", only years in which each casting was produced for that particular year's production.