Quote:

§ 478.34 Removed, obliterated, or altered serial number. No person shall knowingly transport, ship, or receive in interstate or foreign commerce any firearm which has had the importer's or manufacturer's serial number removed, obliterated, or altered, or possess or receive any firearm which has had the importer's or manufacturer's serial number removed, obliterated, or altered and has, at any time, been shipped or transported in interstate or foreign commerce.

I wouldn't sweat it.  Missing a digit isn't an attempt to remove, obliterate, or alter.  Might be an issue if you try to sell it but just owning it shouldn't be a big deal.  If it really bugs you the ATF can electro-pencil a new one on or peddle it to the Mexicans for you.