The 1954 trucks were carried over for part of the 1955 model year, as the new, redesigned 1955 trucks weren't quite ready. These early versions are know as the first series. Essentially the same truck as the 1954 model, it came with a Bombay Ivory painted grille with Onyx Black Chevrolet lettering. The bow-tie emblem was striped in white rather than red. The hoodside emblems were a differnet design, with Chevrolet lettering beneath the numerical designation. Mechanically, the biggest change was the use of an open Hotchkiss drive shaft rather than the previous enclosed torque tube type.
On March 25, 1955, the Advanced Design era ended as the new Task Force trucks were introduced.
(from the Chevrolet Pickup Red Book)
I would guess that there would be more then 500 of the 1st series trucks manufactured due to the fact that in 1954 Chevrolet was putting out about 25'000 trucks per month.