I suspect that it could have been the original transmission. They used the 833OD up to at least 87 or 88. It was a very uncommon option, but I've seen them.

If that truck is like the one I took apart several years ago, it will have a hydraulic clutch. The bellhousing will be a multi-pattern so other transmissions will bolt up to it. It will also have a 143 tooth flywheel.

It is certainly an unusual truck. To bad it's on the opposite side of the country!