Here is a 1954 GM (Chevrolet truck) accessory mirror arm used on large trucks. This style is a telescoping, single arm, with a rectangular mirror. Other styles were available (round mirror and solid arm, etc).
Here are more photos of 1954 mirrors. The drawing shows a light truck with standard arm and standard round mirror, a 1.5 ton truck with standard arm and rectangular mirror, and a COE with telescoping, single arm and a rectangular mirror.
Here is information on several accessory mirrors for 1954/55 (Z series) GMC. They offered accessory mirrors on fixed arms, telescoping arms, and had a brace-arm (as a separate part). And, round or rectangular mirrors were available. I do not see a dual-arm brace but I thought they had that available.
All these mirror combinations (Chevrolet truck and GMC) were accessories. An owner/dealer might have ordered and used any style arm/mirror/brace on any size truck. I'll check my COE cab against my 1/2 ton cab to see if the mirror holes and inner support/strength brackets under the dash allow all size arms on all size trucks (I think they allow this).
