Now that I have had a moment to compose myself...
I don't know what caused this to happen. I did not notice any rust... So I went outside and your keen eyes are correct. There was rust at both of the weld joints. I am going to check the other side as well. It seems that the bottom was not welded as good as it could be....
I am not sure what caused this to finally give way, I wasn't doing anything out of the ordinary. Maybe a stump got up in there and I twisted or turned.. I know I did a PT pucker earlier picking up a large stump and having the back end turned. Maybe that stressed it...
Getting out was a little hairball. It turns out that the steering on the PT works like this. One cylinder activates, then the other. So, by going slow I could still turn the tractor, although you had to think ahead, and turn the wheel a bunch to get it to activate the connected cylinder. Of course this was doing trail busting so I had stumps, limbs and hills to contend with. And the other cylinder (the broken one) to look after so it did not hang up on anything..
If this is a bad weld, it is the second I have had to chase on this tractor. Granted, it sat outside for nearly 5 years rotting so who knows on all of this...