Mark,
You most likely have correctly concluded that the mirror was not put in the GM-intended position. The GM outside mirror-arm on the passenger side has
an odd bend to it.
The holes for the GM mirror arms are in similar locations on both sides of the cab. If you look under the dash, you'll see a small brace with holes in it - those holes are where the mirror holes are drilled
The driver's side mirror was standard equipment. The passenger side mirror was a factory option, or an accessory installed by the dealer/owner. Sounds like you have a home-job right-side mirror on your truck?
