I can't figure it out either, but I do have an idea. Enjoy it. The flat will help cut way back on the glare. You will be the only one (well, besides all of us) to know that it was not intended to be that way. Proceed and enjoy is my idea FWIW.

1947.2 GMC 1/2T SWB panel
1947.2 GMC 1/2T long bed
1948 GMC 1/2T short bed