Don’t know how close we’ve come to finding a concrete answer John, but we sure got a lot of opinions. It’s kind of hard to tell what was original when people post pictures of their trucks. As I’ve said way to many times, I’m very fortunate with the 1950 and the 1954 trucks that I found. I’ve met and talked with the original owners of the trucks and I got the trucks within months of them selling them. I’m about 90% confident that I know their history, the other 10% I chalk up to the PO’s memory failure. But when you’ve been around these trucks for a time it’s pretty easy to spot what was original and what was repair or aftermarket.
As Alvin pointed out, you can drill and tap a hole in the center post, screw the mirror in and lock it with a jam nut and no one will be anymore the wiser. He’s the only one that will know whether it came that way from the factory or not. Now if you take your tools to it and do a little dismantling you will discover how it was done. That’s when I feel like a Jr. archeologist on a dig. My buddy with the ‘50, 3100 had his mirror with the old ’Z’ bracket mounted where the 5th hole should have been. Upon dismantling it I found that someone had just drilled and tapped an 8-32 hole thru the garnish and into the reinforcement. It was clear that the reinforcement was never drilled from the factory.

Although I can’t find it noted anywhere in the manual, I don't doubt Bill's interpretation of what RD means, it’s the only thing that makes any sense to me. There were a lot of these AD’s exported to countries that had right hand drive. But typically as you observed, in the GM parts books Right Hand Drive was represented as RHD.
Denny Graham
Sandwich, IL

Last edited by Denny Graham; 04/26/2011 2:58 PM.
Denny G
Sandwich, IL