The 1947-1955 Chevrolet and GMC came from the factory with left mirror arms on left hand drive trucks. This was particularly important if the truck was to have a large bed that covered the rear window.
To keep sale price low, the right mirror arm was an accessory (dealer installed). It was very important that the dealer place the mirror just right so the actual mirror was viewed by the driver through the lower right corner of the windshield.
To prevent dealer mistakes, GM placed an inner panel in all trucks covering the area were the two holes would be placed. This panel had factory holes, showing the mechanic where to drill. Thus, two exterior holes could always be in the proper place. Yes, the glove box liner would be removed to make room for the drilling.
Inner panel guide for drilling (above)
Outer panel before drilling (above)