That crossbar sure looks familiar, as I had one of them about 30 years ago! NAPA auto stores around here used to sell those crossbars empty and they marketed them as accepting most any type of beacon or roof mounted lighting. The feet have a quick release clamp that goes on the raingutters and the amount of tension generated by the positioning of the feet held the bar to the roof when it was clamped down. Since that truck doesn't have gutters, those clamps have obviously been modified. The bar was very lightweight and was made of aluminum, and could bend or bow fairly easily under a particular situation.