Jim,
HO52 and HO72 are 5200 lb capacity and 7200 lb capacity.
I believe that the difference is going to be primarily in bearings and brakes, but one could argue that a 4.10/11 gear set is decidedly less adequate in the grunt department than a 5.14 gear set. My assumption would be that is a factor in the capacity as well. Generally speaking though, HO52 is 3/4 ton and HO72 is 1 ton.
As has been illustrated in the previous posts, 4.10's and 4.11's were made for Eaton axles. The 3rd members will swap between the two. As far as I know the axle spline counts were always the same. There will be mechanical differences between the 3rd member assembly from year to year, but they should all bolt into the housings and mate up with the axles regardless. I can't speak for the Spicer axles. Just to be clear, these early axles are Spicer axles, and predate the purchase of Spicer by the Dana Corp. So if you are working on one of those, you'll see them referred to as Spicer in the books.
