I was walking around AutoFair at the Charlotte Motor Speedway today looking for stuff for my '65 and came across a GM Utility Tray which I've seen pictures of but never one in the wild. It was listed as a tray for a '55 - 59 which it may very well be since there is a slight radius to the back that bolts to the back cab wall above the gas tank. But it will work in my '65 without too much trouble. It was listed for $120, I offered $100 and the guy said he'd be glad to get rid of it since he's been hanging onto it for about 30 years. I didn't find what I was looking for, but picked this up along with a nice original grill for my brother's '55.2 Chevy 3100.
The tray itself is in really nice condition but they probably don't get much abuse sitting behind the seat.