I have seen them mounted on the inner fender (other people have posted finding one on the inner fender), but that is not a common location.
As a factory option, they would have been mounted on the intake manifold.
As people often post, these were work trucks. Many were bought without an oil filter, but an owner might add one if they obtained a used one off any other vehicle.
A used filter might not have long enough oil lines needed from the rear of the block (Chevrolet) to an oil filter at the front of the intake manifold. I have a few old by-pass oil filters that have one line that is too short to reach to the front of the intake manifold. These lines would work with an oil filter mounted on the inner fender.
