Very interesting and informative. Here are some comments and questions:

- 3106 is Carryall Suburban with panel-style doors
- 3116 is Carryall Suburban with end-gate (not an option, a different model)
- tail/stop (and signal) side lamps were an accessory that was required in certain states - I's say it was uncommon overall. Does your truck have built-in turn signal mechanism?
- What is the code on driver's door-jamb ID plate?
- What is the engine serial number - pressed into the block at the rear of where the distributor enters the block?
- Stainless around the side door windows was not available in Suburbans (I think) but I think (as you do) that the stainless trim was standard around the windshield and, I thought, around the panel-door windows.
- Your Omaha Orange on inside end-gate theory seems plausible. Maybe someone with an earlier end-gate will post a comment?

How about posting link-s to photos? Greg Hill might like to see the grease pen markings (his 54 pickup has some).