I would say that there are two questions at least:
1. Does each individual "axle model code" align with a unique and single "plant code"? In the example used here, is the WES-M always a 936? If yes, for all years, several years, or a certain model year?
2. Does someone have a list that cross references these two codes?
Since both codes appear on the same tag, and since that tag was applied at the axle assembly plant, it would only make sense that both codes were applied at the same time. This causes me to think that there is a one to one unique relationship between the codes.
But why have two codes? To avoid errors, a 936 is easier to remember than a WES-M. 999 permutations with 3 numbers compared to 358,800 with 4 letters. Of course adding the "A" to the 936 adds to the equation, but only if there were also a 936B and others.
I agree it would be interesting to hear from Kevin or Jack on this. I would think that getting a replacement tag for a 67 GT500 or 69 B9 would be way more popular than a 66 Bronco with an I6, so they may simply be using reference data from known examples.