Factory rear view mirror confusion

[Mister-B]

In '69, getting a rear view mirror was an option, believe it or not. Mine didn't come with one, so a PO installed a glass mounted mirror, instead of the factory type that attaches to the roof of the cab.

Here's my question. I was at the junkyard today, and they had a 71 gmc and a similar year chevrolet. Both of them had roof cab mounted mirrors, but the actual roof of the cab had a hump in the metal lining. Almost like it provided extra space for the screws. My truck doesn't have they hump though. It is in the pic below.

So, if I want to buy a factory type mirror, how will I be able to do it if my roof doesn't have the hump?



Another shot after I removed the cheesy covers from the visors lol

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