Did you raise the hood? The cab mounted serial number data plate should be mounted on the right front side of the cab cowl (facing the side of the truck, not the front), which is covered by the rear section of the right side hood panel. Pic attached to show location.
Too hard to tell what series the truck is by dimensions as both the 4000 and 6000 series trucks had both short and long wheelbases (shown in the 2nd and 3rd pics).
As to the brake assessment, back ones are definitely out of operation with the wheel cylinder feeder lines cut and crimped closed.
