I don't think it was a fire truck because in those days, fire trucks didn't have air over. They didn't want to wait to build up air pressure. Now larger fire equipment has straight air, but they keep them hooked up to a regulated air line to maintain 120 psi.
For the purpose of a truck tractor, the air over and 2 speed was a must. I never cared for the big window because it was a magnifying glass in the Texas sun. The shade between the door and the back window was the only cool spot in the cab. It is quite likely it was ordered by an owner operator or a small fleet because then as now, drivers liked chrome.
