A guess (and that's as good as it gets) is a Mex Mex X Thayeri at least that's my opinion but it could be a cross involving any/all of the mexicana complex (or even other lampropeltis). Unfortunantly ignorance has nothing to do with hybrids exclusively. Mis-identified animals are commonplace, hybrids can muddy the waters somewhat but I don't think i've ever seen someone I would buy from (meaning a knowlegdeable, reputable breeder) representing an unknown animal as anything but...
Someone who would think that's a grey band would most likely identify a pure mex mex or greeri as one too.
Horridus