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1976 F100 thru F350, P & M Series Owners Manual
It could be Ford switched Main/Aux designations in 1977. Rear might have become the auxiliary then.
1976 Owners manual shows below:
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Slideshow: Called the Fortress, the 850-horsepower pickup combines Raptor underpinnings with military-inspired features, survival equipment, and a starting price of $285,000.