Bill wrote "Serial numbers can be removed and reapplied legally. Maliciously removing them and leaving them off is illegal." Bill, can you cite a source for that?
Here is what the law says:
"(k) It shall be unlawful for any person knowingly to 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 to 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."
There is no mention of intent or maliciousness; the only copout is "knowingly", so if a person had no idea where the serial number was or what had been removed, there would be no crime. FWIW, Section 921 exempts antique (pre-1899) firearms from the whole chapter.
I know BATFE's predecessors used to allow a gunsmith to replace numbers removed in the course of repair or customizing, but I don't see any actual authority for that.
Nor is there any mention of the date the gun was made or whether serial numbers were then required or not, or any mention of multiple serial numbers; the law seems to assume there is only one serial number.
In reality, I can't see BATFE worrying about an old gun with one of several serial numbers removed. But the question was whether the gun could be a problem, and AFAIK, the answer would be yes, technically, it could be.
Jim