The Firearms Blog has a story of an ATF determination letter that the Plum Crazy brand of polymer receivers from a few years ago are non-compliant to he law in that the serial numbers are easily removed.
During the examination of the submitted AR- 15 type firearm receiver section (serial number RM00501 ), FTB also found that both the height and depth of the serial number markings are acceptable and thus compliant with§ 478.92. With respect to the adequacy of the attachment of the insert for this frame, the FTB evaluation revealed that the serial number consisted of a thin sheet of metal-like material and was attached to the surface of the embedded metal insert. This serial number was easily removed when a hammer and screw driver were used to peel the sheet off of the metal insert. This action took approximately 1 minute and caused no damage to the receiver. (See enclosed photos depicting the removal of the serial number insert.)
… the method used to attach the serial number insert (serial number RM00501) on the partial receiver sample is not compliant with Federal Law.
If you have a Plum Crazy lower receiver for your AR-15, purchased prior to 2011 when the company abruptly ceased production, our advice is to hide your dog.
Seriously, it might be a good candidate for a boating accident.