The trucks system of self loading its own trailer is very similar to the BCFP system for self loading there off highway trailers . I was working for a Gypo on their booming grounds summer of 1965 , they had an late 1950's or early 1960's Intrenational highway truck rigged up as a logging truck that loaded it trailer in a similar manner. We use to just slack the line down to the deck of the truck when loaded , but can't see the line in those pictures ,so I guess they unhooked it every load .