Nice find.
On the speedo needle color. Either it was changed out at some point (not likely given the history you stated), or GM used leftover parts on the big trucks.
I know that Ford back in the Model A era used a lot of leftover parts on their larger Model AA trucks. I had a cab off a '30 AA that had the quarter turn gas cap, but it was a '29 style "Crackerbox" cab. The '28-29's had threaded gas caps.
