Sounds as though there was a hoist outfitted to your chassis at one time but has been removed. Your assumption of the PTO operating a hydraulic pump is correct but it seems the cylinder(s) and or hoist mechanism has been taken off and the body U bolted to the chassis. It would be common to leave the hinge attached in such a removal. You will likely find holes drilled in the frame where the cylinders attached. If the body longsills are sitting directly atop the frame then chances are it had twin cylinders attached to the frame and the body. Look closely at the longrails and you may find evidence of that. If the hinge is incorporated into a subframe then it was likey to have been an underbody twin arm or scissor frame hoist wherein the cylinder(s) and mechanism were also included into the subrame. It would not be necessary to remove the PTO from the transmission and it may still be useful.

1953 Chevy 5-window 3100
In the Stovebolt Gallery
More pix on Picturetrail

Dave
Engine & Driveline Moderator

If you can't make seventy by an easy road, don't go. ~~ Mark Twain