Supposedly at one time of value equivalent to a pound of silver. https://www.bbc.co.uk/news/uk-politics-26169070
And the Guinea, originally a gold coin. https://en.wikipedia.org/wiki/Guinea_(coin)
IIRC you'd still sometimes see professional fees quoted in guineas until the 1960s. It's archaic now.
To settle this, I thought I'd break out Bob Fisher's tome. The photo on p20 is captioned "the 100 pound cup".
The Cup originally belonged to the RYS and https://www.rys.org.uk/yachting/americas-cup describes it as the £100 cup.
I was hoping the Garrard & Co website would have something to...