I've rebuilt 3 or 4 of these in the last couple of years for our tractors and have seen examples of both of your pictures, so I'll offer my 2 cents (mostly because my opinion is not worth much more than that). 1st, I doubt very much that the cuts on the direct drive splines are the result of the direct drive clutch slipping, rather I would suspect that someone somewhere along the way has had a tendancy to drop the clutch, or shift the O/U under very heavy pulling loads. In any event, there is a lot of useful life left in those splines , and as expensive as a new shaft will be, I would put it back together as is and not worry about it. Not sure what you pulled it out of, but counting the marks tells me it's not out of a 2-180 or 2-155 with the heavy duty O/U because you only had 6 friction plates in it. If you're still worried about it then consider replacing the direct drive clutch plates and frictions with those from a 2-180, the heavy duty O/U has 7 frictions and thinner steel separater plates. This adjustment will more than likely get the alignment of your teeth such that they won't be riding in the same worn grooves on the hub. This will also be expensive, and unless its a tractor that you're planning to run a bunch of HP out of, then I doubt it would be worth the expense.
2nd, on the shaft where the sprag rides, the one from our 2-180 had a large amount of material transfer from the sprag to the shaft itself, so I put the shaft in a lathe and used emery cloth on it until the shaft was polished smooth again. Measured the shaft and it was all within spec tolerances (shaft had not worn it is tough as all get out, just the sprag had started breaking down and transferred onto the shaft), so I put a new sprag in it and it works perfectly.
3rd, replace the sprag. Costly, but its just not worth taking a chance while you're there, IMO.
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