You've seen them but you don't know a '54 from a '60. This may help. I'll keep it as simple as possible.
The picture of the '54 on the left, and '55 first series, had a slanted windshield pillar between the front windshield and door. They were identified with the letter C and the number 1 (C1). The second series '55, and the '56 model had a post that was basically straight up and down. They were identified with the letter C and number 3 (C3). This would be the basic standard design until the cab was redesigned in '61. The picture on the right is a '59. Starting in '57 they changed to the letter D and a triple digit numbering system as in 100 for 1/2 ton, 200 for 3/4 ton, etc., (D100) (D200) for weight capacities. All '57 through '60 trucks used the D + 3 number system. However, on the ID tags the '57 used the letter K, the '58 used L, the '59 used M, and the '60 used P. This identified the year. Behind that letter was a number, either 6 or 8, which identified the engine. 6 for 6 cylinder and 8 for 8 cylinder. On the fender the emblem would read 100, 200, 300, etc., but on the ID tag it would read K6 D100, or L8 D200, etc.. The '54 and first series '55 doors would interchange with each other. The second series '55 and '56 doors would interchange with each other. The '57 doors looked the same as the '55, '56, but had a updated latch system and handles and will not interchange unless you changed the latch system on the cabs. All front fenders from '54 to '57 were the same. In '57 they changed the nose piece and grille and went from the 3 piece side opening hood to a 1 piece hinged hood. In '58 they changed the frontend sheetmetal and altered the outside door skin a little. All '58 through '60 doors and frontends interchange. The grilles and emblems are slightly different. All second series '55s through '60 windshields interchange. Interiors and dashes were all basically the same from '54 through '57 with slight differences. In '58 the dash panel was slightly different from the previous years. In '59, the dash panels were redesigned and are the same for '60. Now having said all this I will say that there are a few (very few) '56 pickups with the slanted post as the '54. I've seen 4 so far. Since they were '56s, I don't know if they were C1s or C3s, but probably C1s.
The '54, '55, and '56 all had the same basic frontend sheetmetal and grilles. All had a 3 piece side opening hood. The emblems may be slightly different. Some later model '56s had the double winged "Forward Look" emblem on the fenders.
The '57 had a new nose piece and grille. 1 piece hinged hood. Hooded headlights. Different emblems. The fenders interchange with the previous years but that is all.
In '58, they changed the frontend sheetmetal. Different grille, headlight package, altered hood and fenders. Hidden louvers under side hood emblems. Different bumpers.
In '59, minor grille change and new emblem above grille on hood. Louvers on the side of hood and slightly different emblems than the '58.
In '60, slightly different grille and emblems. No side hood louvers. All '58 through '60 frontends and grilles will interchange. Some tutone paint jobs had a stainless steel trim package that started at the side of the front hood emblem and wrapped around the fenders, above the headlights, and went straight back to the doors and past in all 3 years.
In '57 the civilian 4 wheel drive was introduced and the letter system was W instead of D, as in W100, W200, etc.. The visual difference, other than the obvious 4 wheel drive, was a flared fender around the wheelwell opening. All W model pickups, Town Wagons/Panels ('57-'66), and higher tonnage D models through '60 had this feature.