I'm actually really disappointed this wasn't addressed with a fairly obvious "checksum" mechanism - Including the ":" in the center, and giving that it's own lenticular pattern, such that only when viewed from the right angle do you see the : and otherwise you just see numbers.

(I'm not sure if this is really plausible - Are the numbers close enough that that would actually work? If you're off to the right or left, or too high or too low, could you potentially see the : with a different set of numbers than intended?)