hi hank. i think that was a dealer installed option. i gues they could put them any where, but most seem to be on the door. the one on my 56 is on the fender and like it there. plus won't come loose from opening and closing the door. 
From what i have been able to find out, they were mounted on the doors if ordered from the factory with mirrors. If not then most dealers mounted them on the fenders. The fenders is where mine are mounted too.First question, I have a '55 210 and the driver and passenger mirrors are located on top of the front and right fenders, not the doors, is this a stock option or was this done after market? The guy I bought it from said it was an executive model and why the mirrors were where they are but I haven't seen others in that location.