A whole lot of going on with those guys lately. So they just decided that the polymer AR lowers formerly made by Plum Crazy (now New Frontier) are illegal. (Link to the article under the copied text)
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.