I think you are opening a can of worms that doesn't need to be opened. The 1981 letter concerns reproduction stocks for Lugers and Browning Hi-Power pistols. The 1999 letter addresses Canadian Inglis produced guns outside of the purview of the 1981 letter. The 1981 letter grants legality to reproduction stocks for the Luger and Browning Hi-Power pistols. However, the 1999 letter specifically does not extend this exemption to reproduction stocks for the Inglis guns. It in no way removes the exemption for Lugers and Browning Hi-Powers granted in the 1981 letter.
If it is made after 1918...it is a reproduction.