Here are two photos of a right side 1970 DeVille mirror.
Sorry that the mirror is dirty, this was taken on the first day I bought my car and it was a rainy day.
I took the photos because I never saw a Cadillac mirror with holes in the base like this. My other cars that have passenger side door mirrors are all cable remote mirrors and mount to the door with two studs that pass through the door and two nuts and a bracket inside with the cable. You have to remove the door panel to install or change them.
Looking in these holes, I see two Allen set screws, so I am assuming the bracket screws to the door and the Allen screws secure the base to the bracket.
I have been told this is a correct and rare mirror, and since 69 and 70 are almost the same car, this is probably what you are looking for. There's one on ebay right now, but not in great shape.
Good Luck,
Brian