I can't be as gentle as the others in my warning Bob... IMHO From what you have described, I envision that the serial number on the front of the lower portion (or grip frame) and the serial number on the left front of the upper receiver or barrel extension has been removed. Regardless of when this happened, this gun has had it's serial numbers "defaced" (see my attached photo to see what those areas are supposed to look like) and the act of doing so is a felony, and mere possession of such a firearm may also be a felony. If the areas where the red arrows in my photo example are where your grind marks are... back away from this Luger...fast.
My stongest advice is that if you have a legal paper trail as to the origin of this weapon, then my advice is to go get your money back... and if you DON'T have a legal paper trail as to the origin of this Luger, go TRY to get your money back... Owning this particular Luger just isn't worth a black mark on YOUR record that could ultimately cost you your right to bear arms FOREVER for a felony conviction.
If the grind marks on your Luger are in these two locations, there is a very good chance that the gun is stolen... and any real firearms dealer would not only turn you down on a trade or sale, but would probably turn you in for trying to do so... Play it safe Bob... Find another Luger that hasn't been messed with.
Best of luck,
-John Sabato
http://boards.rennlist.com/upload/numlocation.jpg