This is probably more aligned with the later service replacement since it is from the 1975 master parts catalog. I'm sure someone else will offer to compare with an earlier version and advise us if there is a more correct number.
The number 17682 applies to the "mirror assy - rear view outside - includes gasket" part.
For the 68 Mustang, it looks like from page 2665:
1. The original left hand manual mirror retangular head was part C8AZ-17692-B.
2. The original was replaced by D4AZ-17696-A which says it works for both right hand and left hand applications.
With the original at revision B, earlier MPC's might clue us in to whether or not the factory installation was an earlier rev. "A".
Switching to use on other cars, the first clue is that the third character is an "A". Something designed specifically for the Mustang would have a "Z" (but of course a "Z" part could be used on a Cougar as well, or later on for a Maverick/Comet)
Doing a text search has a few results, as can be expected.C8AZ-17692-B is assigned for:
1. 68 full size Ford (A)
2. 68 Mustang (F)
3. 72 T-bird (S)
4. 67 Bronco (U)
Hope this helps