Thanks, Brad

As you know, Chevrolet did have different model numbers for various configurations but those numbers were not placed anywhere on the truck. I cannot find a chart/list/link for GMC model numbers for different configurations within a model-size. Here is one GMC chart of Conventional Model Specifications (for 1951, I think - from Rob English's oldGMCtrucks.com website).

A bill of sale (receipt) with model number is the only thing I can think of that might "officially" ID the configuration.