What year is your truck? My 1st series '55 did not come with reverse lights and I don't know when GM started using back-up lights as standard equipment. It is possible the switch you have is a neutral/park safety switch.
You can check this out with an ohmeter...check resistance across the terminals with the shifter in neutral and park. If you get 0 resistance in these two positions, then you have a safety/neutral switch. If you get zero resistance when the shifter is in reverse, then you have a reverse switch.
