I bet/vote that it is a custom modification (non-GM option).
That is what I was thinking as well, but this truck is somewhat a barn find. It don't appear to have ever had holes to mount the spare tire bracket or a tire well on the left side. The holes and well could have been filled in but it was done perfectly and it was not done very recently. That don't explain why the right side tire well is an obvious and poorly done modification, but who knows, I guess anything is possible.
