Interesting. Maybe they got a variance, or something has changed lately.
Title 27 CFR chapter 11 part 478 (478.92 a1 i and ii, as well as 479.102 a 1 and 2 say this:
For firearms manufactured
or imported on and after January
30, 2002, the engraving, casting,
or stamping (impressing) of the serial
number must be to a minimum depth
of .003 inch and in a print size no
smaller than 1/16 inch; and
(ii) By engraving, casting, stamping
(impressing), or otherwise conspicuously
placing or causing to be
engraved, cast, stamped (impressed)
or placed on the frame, receiver, or
barrel thereof certain additional information.
This information must be
placed in a manner not susceptible of
being readily obliterated, altered, or
removed. For firearms manufactured
or imported on and after January 30,
2002, the engraving, casting, or
stamping (impressing) of this information
must be to a minimum depth of .003"
Does your pistol have a born on date before that? Like I said, the regs could have changed lately. They do that a lot. It's also possible that those particular parts of the regs have "sunsetted" like a the so-called assault weapons" ban.