hmmm, you mean are you okay with that pistol grip that you had on your pre-ban M1 benelli with an extended mag tube.. why yes, you are.
The thing with the pre/post ban is that your gun had to assembled as such (by you or the manufacturer) before
the magic date in 1994.
If you did not already have the PG on your pistol, however, you could not add one today.
How the ATF would go about proving someone did not have a certain weapon in a certain condition is the real question. Obviously, if the parts used or the gun itself was manufactured AFTER the magic date, it would be foolish to alter a weapon. Otherwise, it would just be illegal (like speeding, but a felony