There is an historical arc to the length of pickup beds. They started out very short; little stubby appendages on the back of a Model T. How long was it? Four feet? Over the decades, they grew and grew, until the 8′ bed became the standard (9′ stepside beds were available on 1-ton trucks, but quite rare). Then in 1969, Chevrolet created an extended length 8’6″ bed, specifically for hauling longer slide-in campers, and dubbed it the Longhorn. It came in both 3/4 ton (C20) and 1-ton (C30) versions, and since the badges are missing on this one, I can’t tell which it is.
Here’s that additional 6″, obviously grafted unto the 8′ bed.
And a view from the inside.
Oddly enough, Chevy dropped the longer-bed Longhorn after 1972. Ford then picked up on the extended-wheelbase camper-special truck theme the following year, 1973. But they went about it differently, pushing the rear axle further back, but not extending the length of the bed. The resultant F350 Super Camper Special (CC here) had the same benefit.
And now 8′ beds have become scarce, except on the biggest trucks and a few fleet specials.