If you don't have the original lower tang, do you know the original number? Or does the side of the upper tang have the number on it? If memory serves me (not often!) they were also marked on the upper left side tang.
I had a Winchester 1885 that the lower tang was cracked. I had the crack welded, and polished it out, losing the factory engraved serial number. I had an engraver friend just cut the old number back on the tang in the same spot, and font style. Mine was an early antique 1885, but I still wanted the correct serial number on it, in case some future owner wanted to letter it.

Without the serial number it may or may not be made after the cut off date. To be on the safe side I would require an FFL if I were not selling it face to face.
You can always count on the government to be unreasonable.

40 Rod

You can sell to a holder of a C&R License also.

Don G

There are lots & lots (maybe literally a million or more) of post-1898 rifles & shotguns out there without SNs.  Before 1968 many low-end .22 rimfires, bolt actions in particular, and especially those made for Monkey Wards, Sears, Western Auto, etc., were made without numbers. Many circa-1950 Marlins have none. How many Remington-made military rolling blocks have serial numbers (and not just assembly numbers)?  The post-1902 No. 5s have none.

We have been discussing low end Stevens SxS shotguns.  Do those and the old break-open single shots have SNs?

5 years back, there was a much-modified coil-spring Low Wall with a SST making the rounds.  IIRC, the lower tang bore the number 1492.   

Are those of us who know of such things criminals?  There must be a process (or more than one) for disposing of such things.  What does the FFL do?  Simply enter "NSN" or a code for "none"?

It has not been ground off or altered, it has been removed by changing the lower tang. I would ask CPA first. Technically the BATF would probably need to make a ruling on it since it originally had a serial number on it. I am not sure I would want to stir up that hornets nest. If CPA asked the question as to what to do if a customer requested such a modification it might be less of a problem.

I have  HiWall with the serial number polished away.  I applied to the BATF for a serial number.  They issued a new one.  I has to be installed on a permanent part of the receiver, not a removable tang where the old one was.