It's a computer desk for my truck. See, Greg was really close! Take a look at the first picture...the big "tabs" or "ears" on either side slide down between the center "seat" and the driver and passenger seat on either side. Then the "mystery hook" anchors the desk in place via the seat belt. The hook is offset because of where the seat belt buckel wind up when all the slack is taken out of the belt.
The three sided wall on top is there to make a place for maps, guides, pencils and such. Once the desk was delivered I realized I could have moved that back toward the hook and had more storage space. That's the one design change I would have made.
I'll post a demensioned drawing under another thread. By making your own measurements a good metal shop can whip you one up.
Thanks for playing along! It sure was fun...and the collective imagination of this forum is something to behold!