Where is serial number on crosman 130?

Crosman made the model 130 in two variations the first one had walnut pump and handle 1953-1954. The second version had a metal pump 1955-1970. For a time Crosman did not put serial numbers on original parts. ( I had to do some reading to find this fact out.) Yours may be one of these models. I also own an old Benjamin that has no serial number.

Reference Blue book of airguns 7th edition.