Someone on here in past recommended these guys. I have no connection to them and haven't used them, just passing this on.
Import Service I noted on their website "federal law requires that all firearms imported by licensed importers be engraved with the importers name and the city of importation, regardless of the guns value. Your gun will be laser engraved precisely at the minimum specified height and depth: 1/16 and .006 inches, respectively." It surprised me as I have never seen it on a vintage SxS before. Is it correct?
AFAIK, the "good", "professional" importers will put the engraving in a good, legally-sufficient, inconspicuous-unless-you-know-where-to-look-and-won't-diminish-the-value place.
The requirement for engraving has not been there since the beginning of time, so it is possible on guns not so engraved that they came in before it was required. Or, OTOH, someone merely forgot to do it and no one noticed.