I only had my broken and badly repaired 72 valance to compare but....
Notice the upper bracket on the 73 is larger and the hole is set forward, the front lip below the grill is notched on the 73, whereas the 72 is not. The 72 has a smooth transition along the end where it meets the fender and the 73 has a step.
I suppose the brackets and the metal bumper filler on the 1973 accounts for some of these differences.