Hey Tony,
Your understanding is the same as mine from what is read on page 10 of Garstka's '82 paper on mexicana, stating that all of the living specimens of alterna that were examined had a distinctive silver-gray iris color, with mexicana and ruthveni having a golden-brown iris. It mentioned other lampropeltis that were examined not included in the mexicana group as also having a golden-brown iris color. You will often see reference to mexicana, thayeri more specifically; having an iris color that closely resembles saddle or even ground color of the respective animal. These variable iris colors can be anything from nearly black, to the very lightest shades of colors with the largest percentage being shades of orange, red and that mentioned golden-brown. Look closely at all of your fantastic new thayeri, particularly in close up head photos and you will see this. The only real distinguishing factor I see referring to iris color is that alterna are more uniformly distinguished having the silver-gray iris which sets them apart from the rest.
I also have a number of Tim's F1 and F2 thayeri, they are awesome animals and congrats on taking the plunge and buying a few! Your split saddled F2 male is awesome. We all need to get our hands on a few while that female is producing good clutches. Some of the second, third and so on generation offspring from that female are spectacular.
Mike
