I have two original right hand taillights, one was NOS in the box, and the drain holes are "upside down" compared to the left hand one on an AD pickup. So if you're worried about the drain holes, the left will have the garnish screw at the top and the right will have it on the bottom.
I had quite a few discussions with experts several years ago about this, including Carter and several others, and the only reasoning we could come up with was the fact this taillight design was around well before the AD's were produced. The older pickups had the lights mounted the other end up. Since the right lights were an option and were dealer stocked for several years worth of trucks, GM just never redesigned the right one when the AD's came out.
Bob
