Which 273?

look at the bolt angle. There is a casting line on the intake end that follows the bolt angle, as the 66 is almost perpendicular to the head surface, the 65 is at an upward angle. If it is not evident, look at the bolt holes. The early ones are smaller (hard to tell if you dont have a comparison next to it) and have a angled pad they seat onto. The 66 and later is a flat plane.