F255X
F = Flint Michigan, Engine Assembly plant
2 = 2nd-Series, Production run from March 1955 thru Augusts 1955
55 = 1955 Model Year
X = 3100, 3200 Regular, No RPO = Regular Production Option
On 8 cylinder 55
On 6 cylinder 255, Denotes 2nd-Series 1955 truck models.

1955 2nd series truck, GM Vehicle Information Kit....pdf page 6, actual page 63

1954 Chevrolet 3105 Deluxe Panel Truck
w/Factory HYDRA-MATIC