Kevin,
What I am saying based on the LMA article (and supported by the NACA titles) is that after the standard was adopted, the '3' was added to the '24S-T' labeling to be consistent with the standard. Maybe someone can find a well-dated pre-1948 example of Alclad marked as '24S-T3', but I don't think it'll happen.
added comments: By the way, you say "As all of us who have searched for labeling images can attest to, there was a fair degree of variability in the labels before the war". Isn't it in fact the opposite? I think in all the definitively pre-war examples we've seen, the labeling is more consistent than in the WW2 (or later) examples.