Well,.... "thayeri", known as the "Nuevo Leon" kingsnake, and also commonly called the "Variable Kingsnake" is just that....
extremely variable!, and the same clutch can produce a wide assortment of different looking offspring!.....even the same parents can have totally different looking siblings from year to year...leonis phase, milksnake phase,and any combination in between,.....even solid black(melanistic).
I think the majority of your snakes lineage is "thayeri", but like some of the others already stated, it's quite possible that it has some gene-flow from another source. Most "pure" thayeri that I'm aware of from many years ago, tend to have a more "mottled" look to their head, as well as the light temporal band being strangely shaped, or many times being broken.
I also tend to agree with the fact that many are not a "pure"
representation, as they were some years ago. They have certainly had their share of "mixing" in recent years, along with many other species of snakes as well!
here are pics of two typical unrelated examples of the
"milksnake" phase thayeri from back around '95 when there was no doubt what they were!
In any case, that's my honest opinion as far as what you have.
I'm always bothered by the fact that you can't really tell what you have anymore when it comes to certain species/subspecies with 100% certainty!!......this practice is becoming all too common, and gets nothing but worse every single year.
best regards, ~Doug


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"Better to be silent and thought a fool, than to open mouth and remove any doubt!"