On my '55 E12, the inside rear view mirror bracket never seemed like it fit as a factory part should. It has three screws, but only the two on the sides were used.
The Parts Manual lists a 276536P bracket, inside mirror for the 2R but nothing for the later trucks. There's no illustration of that bracket.
Anyone ever seen fer-sure factory inside mirror bracket for the '55? Got a photo? Got an extra mirror and bracket?
One of the later truck parts books does show the inside mirror for trucks with the one piece windshield all the way through 64. The bracket is the same as used on the (I think) 53-55 Stude cars. It has three screw holes and mounts right above the windshield in the center. It has a 4th hole for the mirror shaft. I have the NOS 298724P bracket that I bought years ago at Newman & Altman in case I ever bought another truck. I have concluded that that eventuality is very unlikely. The bracket cost me $10, and you can have it for the same price.
The truck mirror is slightly different from the car mirrors of that period, but the car mirror (which is easier to find) will fit. I don't have an extra mirror, just the bracket.