I thought I indicated very clearly it is not a valid nomen, apologies if that was unclear, but it is a useful name in that it refers to an easily recognized form of univittata. And you're right of course, we'll never identify what it is with any certainty. But I believe it's fairly safe to say what 'Frostbite' is not, and that is a variegated form of a plant it bears virtually no resemblance to.
And I suspect the seed came from someone's basement. I have several forms of univittata and xylonacantha, I could tell them apart with my eyes closed. I am quite familiar with the bible passages quoted above, HSG may be referring to the heteracantha form, or perhaps another that never found it's way to cultivation. In any case, forms that never found their way to cultivation are likely not behind the lineage of 'Frostbite'.
Here is another Gentry quote from A. xylonacantha, with irreconcilable differences bolded:
Single or cespitose, short-stemmed, openly spreading rosettes with long tapering, freely seeding spikes. Leaves 45-90 x 5-10 cm, ensiform-lanceolate, broadest through the middle, long-acuminate, rather rigid, plane to concave above, explanate in shade, rounded below, green to yellowish green, sometimes glaucous, with or without pale center stripe; margin continously corneous, straight between remote prominent teats, but looping over the teats; teeth broadly flattened, thickly capping the broad teats, frequently 3-5-cuspid, commonly 2-5 cm apart, 8-15 mm long, light gray; spine 2.5-5 cm long, trigonous-subulate, stout, flat to slightly grooved above, keeled below, light gray;
Agave
"American aloe plant," 1797, from Greek Agaue, proper name in mythology (mother of Pentheus), from agauos "noble," perhaps from agasthai "wonder at".
"Some talk the talk, others walk the walk, but I stalk the stalk"
