Serial number, not VIN. VIN's weren't used on Series trucks until the early 80's.
You are looking in the wrong place. It will be at the very front of the front RH spring on the outside of the frame rail in the triangular-ish space above the spring mounting eye.
Many original chassi sdon't have them anymore as replacement of the dumb irons (not dumb metal) is pretty common.
Not sue of the origin of the term. But it is pretty common.
also, for more serial # locations:
Originally Posted by skapegoat
I can take a shot later today, but when sitting on the ground, looking at the frame from aft of the ft passenger wheel. There is the aft front spring mount attached to the frame.
And I looked at the cross member that comes under the passenger door hinges.