Catching up with you guys...
Joe - The first letter in the code stood for the length and, more importantly, the price. Don't have the catalog with me on this trip, but don't think the CS meant anything special. You will notice a similar scheme with Ampex tapes. However, the letter was separate as I recall (or mixed in with the label code), e.g. you will see LCL xxxxxx, L xxxxxx, and LON L xxxxxx with LC or LON meaning London and the L meaning $7.95. By the way, the '4' in front of a number, e.g. L 4xxxxxx, meant it was a Dolby tape.
No knowledge of Cook. Few Command, not as good as Everest nor Mercury from what I have.
Westminister at some point became a subsidiary to ABC and was release by a number of companies including Ampex, GRT, and another unlisted (or Westminister itself).
By the way, in thinking about Stellavox's comments on Stereotape, I don't think they were the same company, i.e. a merger. As the Stereotape name wasn't used between the 50s and 1972 when Magtec started using the name.