Basic Info on Colt New Pocket
I am a serious collector of Colt New Pockets and Pocket Positives. I have 2 New Pockets with 2 digit serial number and 2 more that are below the 200 SN. I own nearly 100 New Pockets and Pocket Positives and have observed thousands more.
None of my early guns nor any legitimate guns I have observed do not have a serial number on the butt. In short all New Pockets that are factory original have the serial number on the butt.
Some Colt factory re-finished New Pockets have had the serial number removed from the butt. I have personally observed two that were factory re-finished that way. They look great, but aren't original. These guns can be recognized by the fact that they have a backwards ampersand or a 6 pointed star stamped on the web of the trigger guard bow, Colt's marking for factory re-work on a gun depending on the era of the re-work.
Also Little John's auction sold a Colt New Pocket at auction about 6-8 years ago that fits your description: a 6 inch blue New Pocket that did not have a serial number on the butt. I have a distinct feeling that if I check my auction records, I will find that your gun matches that one.
I hope this puts an appropriate perspective on the issue.
Also, the gun is not illegal to own as it stands. Before someone makes a statement like that, they should conduct due diligence and contact the ATFE and speak to a true agent and not an clerical analyst.