The confusion at NAPA is probably because there's a slightly different gasket for the differential to housing joint and another for the rear cover. I replaced both of mine when I put the chassis back together (and put sealant on the wrong one as I was putting them on dang ).
Got mine from The Filling Station their PN's FS-54 for the cover gasket and FS-54A for the differential gasket. They were both Fel-Pro brand, but I don't have those PN's.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.