Bear in mind the big window section is available in both steel and fiberglass and plus a person could cut one off another cab. So rare can be misleading. Just last night I was helping a fellow club member move his 1953 cab around that he had put one of the steel big window sections in.
With the kits out there now, I am pretty sure the big window will not mean as much as it used to. The number of them probably grows every year and they become less rare. Anyone with $500, the tools, and the tallent can own one now. Or just the cash to pay a shop to do it.
I think the first year of the Custom Cab was 1955, which included dual electric horns, sound deadening material, deluxe door panels, dual visors, armrests, foam rubber pads in the seats, stainless steel molding on the drip rail, Custom Cab plaque on the door (below the windows), chrome grille strips, color keyed two-tone upholstery and I think door key locks on the drivers door.
Fatfenders56 and 56 Effie are both correct. Custom cab is the extra badging on the exterior side of the door and the dash trim and other interior upgrades. 56 one of the Custom cab options was the Big Window