Farmall Cub

Sliding female spline PTO engaging system wears out shifter lever pin that keeps PTO engaged in 5 hours...(recent phenomenon) Pin wears on one side only. Must be some kind of thrust, but from where?

The thrust is from the input shaft and sliding collar splines being worn at an angle from the original squared off. This angle wants to force the sliding collar back as the shaft turns. The rearward thrust wears a flat spot on the pin, and will eventually wear clean through it.
Only fix is to replace the transmission input shaft and the collar, unless you have access to a machinist.

What is the "Collar" ? Absolutely no discernible wear on the sliding female splined coupler, inside or out ... Really hardened steel.... Some wear on input shaft splines but no trouble engaging output shaft splines. Can do Machinist. (What about pilot shaft on PTO shaft that engages hole in drive shaft ? Could slop create this problem by allowing flex? What should those tolerances be ?) What does machinist do? Square off end of input shaft? Need to square off output shaft ? Then how to compensate for slightly shorter input and/or output shaft(s) ? One last thought.... Could selector "finger" just be too short ? Appreciate immensely your thoughts & diagnosis!

A machinist may be able to build up the splines on the input shaft and recut them square. Probably more expensive and time consuming than buying a new shaft.
It's possible the the PTO bushng may be playing a part in your problem if there is slop in the joint. It should be relatively tight. Tight enough that the PTO will turn from drag with the tractor idling in neutral with no implement hooked up to the PTO. I doubt there is any flex in the shafts.
The splines become worn from people trying to engage the PTO before the input shaft quits spinning. They then just grind the PTO splines into mesh, causing the angled wear, which causes the rear pressure that wears the finger on the lever.

Check to see that the input shaft doesn't have any (or very minimal) front to rear movement. If it does, the input shaft front bearing retainer is probably installed backwards.

Retainer is #30 in this pic. Input shaft is #26
http://www.cleancomputes.com/Cub/Cub%20Parts%20Manuals/TC-37D%20Revision%201%20(Revised%2012-54)/Chassis/Page-88.jpg

Collar is #14
http://www.cleancomputes.com/Cub/Cub%20Parts%20Manuals/TC-37D%20Revision%201%20(Revised%2012-54)/Attachments/Page-134.jpg