Plum was a catalog color in 1967 and from 1988 to 1990. Initially, I confused Plum and Amethyst but Plum never has the muddy brown tint of Amethyst.
This example was made when Plum was not a catalog color.
Blog Update-November 23, 2013 The 1967 formulation of Plum is a very different color, more like Lilac or Mulberry. There is a photo of it in The Pina book (Cool 50’s and 60’s Glass) on page 59.