I thought that I had a JD 31a - but after looking at the stamped plate on the gear box - it's a 31b.
I don't think it's a heavy duty PHD, but it's not a light duty one, either. At least, it's alot beefier than the ones I saw recently at TSC....
Hard packed clay is a bugger to work with - plowing, discing, tilling - just about anything you try to do with it; it's a challange!
Digging post holes would likely be a situation of timing.... not too wet and not too dry. I'd guess that the 31B would probably do as well as alot of other diggers - better than some and not as good as say; a power down, commercial unit.
If the price is right and the gear-box is tight (the only thing that you can really "kill") it's done good work for me the past 7 years.
AKfish