Steve, I believe in 28, early?, the mirror bracket was also fastened to the two screws that hold the visor on. I have a mid 28, and the bracket is fastened on those screws and there is also a picture of a 28 closed cab in "The Model A as Henry Built It", that also has the mirror bracket mounted up on the visor. The door pillar does not have any holes.
The plate was also used on the deluxe delivery instead of D-nuts. Since I do not know the appox date of my delivery body I can not help on when it may have been introduced. The mirror bracket on the deliver is about midway down the window. Here are pic's of the early 30 cab that shows it in the lower position also. Rod