Schrade Pony Express No S/N

[johngalt85]

I inherited several Pony Express knives from my dad, and all of them have the series numbers on the handle below the base of the blade. Except one. I can't find a series number anywhere on the knife, and it definitely isn't where the others are. My understand is that this was a run of 8,000. Was there a second non-limited run of these? Is it some sort of error?