The jack arrangement looks similar to some of our shop porta-power units. The ram is 3 to 4 feet long and it has a channel on the end that fits around the interior top ridge frame. The ram is affixed to the plunger/reserviour and there is a purpose built frame that serves as a base as well as holding the whole works together.
The whole bed cover arrangement is attached at the trailer bed body in a permanent fashion. The attachment framing as well as the construction of the cover (both top and bottom) are purpose built as well. Our unit was labeled as a "PPL" trailer. It came with 3 cabinets, two of which appear to be original and a third that was clearly something added in the field. The original cabinets are forward and bolted to a frame that crosses front to rear over each wheel well. There is some wood decking on the portion of the frame without cabinets. If one were to remove the cabinets and deck the whole of the frame, it could make nice deep seats or perhaps a place to sleep.
We have seen some newer versions of this bed cover arrangement that appear to be made out of fiberglass or roto mold plastic. One of these newer units was a single piece cover that did not expand while the others were expandable.
