I think you will be right to take it back.
I think the under-yoke serial number was removed with felonious intent and ignorance of the butt location concealed by the target grips.

I think there are untold numbers of Smith & Wessons with "assembly numbers" on their paperwork because the butt was covered by target grips, but before they started putting the serial number under the yoke.

There was a case like that here where my friend the Gun Guy at the pawn shop was offered a "scrubbed" Smith and Wesson. He loaned the customer a few bucks and called police and BATF immediately he walked out the door. There was a serial number remaining somewhere on the gun, so they tracked it back to the owner it had been stolen from and returned it to him. That was a long time ago, I don't know if the feds would be so obliging now.