Just bought a '69. Help me identify what. It has a couple unusual additions

[Mister-B]

This is a '69 long bed, with a newly rebuilt 350 V8. Dual headers and new exhaust/tires. Runs really well. Talked the guy down to $1,600.

There are a couple things on it that are a little unusual. I could use your help uncovering the mystery. First, is the spare gas tank in the bed. Is this a chevy installed part, or aftermarket? Second, what is the "U. 9,000" writing on the side? I've seen this on some other brands of trucks and such, but not on this particular truck. Where do you figure this came from? Does it mean this may have been a commercial truck at some point? Maybe an oil drilling rig truck or something?

Here are a few pictures