There are two ways to do this. Way number one your truck came with a dealer mirror and your center trim is drilled as shown (hole is towards top of cab).
https://www.flickr.com/photos/advance-design_parts_co/11590024186/in/album-72157631868452754/You then insert a mirror with a stem like below (several sizes and day/night types were available).
https://www.flickr.com/photos/advance-design_parts_co/8325460715/in/album-72157631868452754/Most have a thin nut backing them all though I did see a threaded trim piece once? Perhaps others will comment on this aspect.
The second way is you do not have that hole and want or have a bracket like this. It screws (standard trim screw hole) in providing a step for the mirror to then bolt to. They are both correct, mirrors were accessories installed by dealers who most likely used mirrors and brackets on the shelf at the time. These stem mirrors were available from the 30's into the early 60's and are still frequently NOS ebay items.
https://www.flickr.com/photos/advance-design_parts_co/34278894970/in/album-72157631868452754/You can see one mounted in a very original '49 GMC I bought in Wyoming some years back.
https://www.flickr.com/photos/advance-design_parts_co/8130452603/in/album-72157631870734114/There is info in the old post that adds additional details.
