It may depend on where you look but Chevy hardcore says They are listed one above the other on the page 3998993: 1972 and 1973 307ci and 350ci, 75cc chambers, 1.72/1.50-inch valves. 3998997: 1972 and 1973 400ci, 76cc chambers, 1.94/1.50-inch valves. Swap Meet Guide To Small-Block Chevy Cylinder Head ID (chevyhardcore.com) I go along with someone sticking a mismatched pair of heads on it back in another of it's lives. Possibly when it went though a valve job with a cylinder head exchange. Speaking of that take a look at the valves and measure the stems. A cylinder head shop that I did head and engine changes in about 30 years ago years ago machined the valve stems down to a smaller diameter so they could save the valve and just use a guide with a smaller diameter hole. They have been out of business for a few years but those heads are still floating around.