There was no one single mirror that was universally installed. The dealers installed:
(1) Ford/Mercury "base" chrome mirrors. See the pic below.
(2) Ford/Mercury "sport mirrors". Usually the Cougar sport mirrors.
(3) US or European aftermarket mirrors, racing mirrors, etc.
The installation of single or dual outside mirrors was not standardized either. By 1971 it was quite common for manufacturers to install outside mirrors on both sides of the car. I don't remember if that was the law however. Many Panteras were equipped with dual mirrors. 6018 was equipped with mirrors on both sides for example. I've seen a few with just driver side mirrors however.
The chrome mirror and the Cougar sport mirror both mount with two sheet metal screws, they don't mount very sturdily, they wobble pretty easily. They hug the windows very closely and don't offer much rearward vision.
Another "period correct" option would have been the '72 - '76 Torino/Montego sport mirror. It used the same "head" as the Cougar sport mirror, but a different mounting base. It mounts to the side of the door with 1/4 inch studs, and is thus much more sturdy than the mirrors mounted with sheet metal screws. It hangs away from the side of the car, and offers a little better rearward vision. Check out the Torino mirror on the Mangusta in the pic below.
