Looks like "3B135103"
Where "3" is model year (probably '83, although could be '73. You'll have to use some context clues on that one, like the front bumper frame horns).
B = Baltimore final assembly location
135103 = serial portion of VIN. This should match the serial portion of the VIN on the cab if all goes well.
If you were to find something on the top of the rail under the cab it should say exactly the same thing.
K