Can't say I disagree. So we basically have three distinct series (or at least 2+1 if the 6 digits and 721s are related) that I see:
Five digit S/N, 2XXXX format (unknown amount, at least 80 in the 22 series and 440 in the 28. Possibly 28k if they are truly sequential but I think that's doubtful)
Six digit S/N, 0XXXXX format (at least 20k guns)
721 + Six digit S/N, 7211XXXXX format (at least 11k guns)