There was an "Open Letter to all Federally licensed collectors of Curio or Relic Firearms", dated Dec 22, 2000, whose "... purpose [of this letter is] to apprise Federally licensed collectors of pertinent regulations of the Gun Control Act of 1968 (GCA). A licensed collector is a collector of curios and relics only and is licensed and regulated under the provisions of 27 CFR Part 178. To be considered a curio or relic, a firearm must 1) have been manufactured at least 50 years ago; or 2) be certified as a curio or relic of museum interest by the curator of a municipal, State, or Federal museum that exhibits firearms; or 3) derive a substantial amount of its monetary value from the fact that it is novel, rare, bizarre, or associated with some historical event, period, or figure. ...."
It is very clear, and I have sent copies of this letter to sellers who were skeptical, along with documentation of the age of the piece in question. Sometimes they shipped, sometime they would not, regardless.