71 and 72 grilles are different enough that they require different front fender extensions. The difference is in the lip inside that carries the chrome metal trim (that trim is different too). The 72 is more shallow than the 71.
They have fender different extensions, but they don't require them. The headlight bezels/grill surrounds don't mount to the fender extensions, and for both years the headlight bezel/grill is out in front of the trim on the fender extensions. The lip on the fender extension is more shallow on the '72's, as is the metal trim piece, but it doesn't effect the grill in either case. There are plenty of folks running '71 grill's on their 72's, it's a bolt in swap.
