I believe that there are 2 different type of mirrors being discussed in this thread. The original question was in reference to the outside door mirror, and the associated door mirror gasket. Brad, you have brought into the discussion the inside rear view mirror, I believe. Please correct me if this isn't what you are referring to. If it is the inside rear view mirror, then the mirror itself is the same for many years, and the only way to tell the vintage is if it is a day/night mirror. Then the "lever" to change from day to night changes design every few years, however, the function remains constant. The mirror bracket that GM supplied for the commercial glass cars is a long neck bracket, however, as you have pointed out, a short neck bracket might be preferred in some instances.
As for the outside door mirror, I believe that GM provided a special mirror for the commercial chassis cars that looks like the GM mirror used on the same year Cadillac, but isn't the same. Only checking the Cadillac parts manual will give a definitive answer to this question as to which years this option was available.