Early 60s 7-1/4 axle shaft seal fix for " obsolete parts"
Possible benifit to it being narrower....
If it installed to a positive stop, the sealing surfaces now rides on a slightly different part of the axle shaft seal surface.
What year are you searching for?
Tapered axle?
Found this, but it doesn't say which specific axle. Says fits 64 dart with slant 6 up engine?
NTN-NS450776