360 Main Bearing difference

I've been looking for a set of .020 under main bearings for a 1973 360. A couple places specified 71-73, and 74 up. On Rock Auto, the part numbers are different. Anyone know what the deal is? I looked in the 1973 Parts Manual and they listed up to 1-15-73, and after 1-15-73 with different part numbers. The after 1-15-73 listed as Heavy Duty. Anyone???