And since the invention of laser inscribed markings, it's now very easy and fast to put additional markings on guns.
It's not necessary to use a roll engraving machine to roll on markings, you can simply program a laser and it'll put on whatever you want, any place you want, any size you want, and do it in seconds.
When markings are roller engraved, the extreme pressure distorts the structure of the metal. Even if the marks are ground off, it's very often still possible to "raise" the numbers with acids or other techniques.
Laser engraving seems to be easier to remove, and since it doesn't distort the metal structure, it may not be possible to recover the markings.
So, possibly S&W is making sure there are plenty of hidden serial numbers where the usual thief wouldn't think to look.