Photo © John F. Ciesla
Passenger baggage was loaded onto them in modular fashion. Then they were towed out to the aircraft and lifted (or hoisted) vertically into the baggage compartment via a transverse belly hatch just aft of the wing, rather than the baggage being loaded in the customary manner through side hatches with a belt loader. (If there was a corresponding belly hatch forward of the wing, I never noticed it.) I only ever saw these used with United DC-8s, and just the Super 61s as I recall.
I went through all the A.net DC-8 photos from the 1960s and '70s, but I wasn't able to find any pictures of these bins in use. The only shot I could find of them at all was in the above picture of a United Caravelle.
Does anyone know the story behind this system? Was it developed specifically for United? Did the planes that used it also have the conventional side doors for baggage loading? Were the bins lifted up with a special piece of ground equipment, or raised with an onboard hoist? Were they then slid along some sort of rail system like a C-130s?