I recently had an email exchange with CRT Customer Service on this very subject. The lass concerned denied any knowledge of the term "Watermate Key" but stated that she had only been with CRT for seven years. She later asked a colleague (who had two more years service) and apparently she had never heard the term either.
It is, however, used in various material on the CRT website and if you Google Watermate Key the first item listed leads you to a CRT publication on the CRT website!
As is so often the case with CRT it appear that it's a case of the left hand not having a clue what the right is doing ................... !!
IIRC the label "Watermate Key" came about as the result of a competition that the then BW held to find a universal name for the thing, with the winner receiving some suitable prize and a term that we could all share and recognise becoming enshrined in the boater's lexicon. Does anyone else have the same recollection?