You can still buy them and they are for light strings to be installed on CXWT which in Canada is Christmas tree wire or Brewery cord as was used on construction sites. Not approved anymore for construction as steel studs kept getting energised by cut insulation. Great for garden parties. Most of this stuff has been replaced by manufactured light strings. Can be a handy speaker wire as the cord was usually #14 0r #12 fine stranded.
Tricky to polarize too.
