If for some reason you can't get it hydro tested, I wouldn't fill it and keep it around the shop or transport it in a vehicle. Maybe I'm overly cautious and given to wild imagination, but imagine something like this:
You're prudently driving down the highway when some [insert stereotype of dangerous driver] slams into your vehicle. The accident is so bad the tank goes flying out of the vehicle, hits the curb, and becomes a rocket. It ends up crashing into the daycare and takes out 1 teacher and 3 kids. The accident is not your fault, but you were driving with a tank under pressure that wasn't "up to snuff" and I'm not sure a jury would absolve you completely. Not sure what your insurance company would say about it, either.
And there's the very real risk that the tank is flawed, if not from the stamp then from some unknown issue. The vast majority of tanks pass hydro every time. But not all of them. No telling if this is that one tank....