The Phantom 377L uses a 52cm Deep Tuttle fin with a 78 cm daggerboard on 298 litres of volume with bat tail. Totally different design to the Equipe in the fin area of the board.
http://www.star-board-windsurfing.com/2017/products/boards/phantom-raceThe Exocet RS Race 380 (311 litres)- this looks like a beauty of a board, uses a 46 cm or a 54 cm Tuttle Box fin and an 85 cm daggerboard with an option for an XL daggerboard.
www.exocet-original.com/en/rs-380.phpThe trend seems bigger sails and bigger daggerboards and bigger fins and bigger volume.
Returning to the Mistral Equipe: according to some earlier posts by other learned commentators on seabreeze:
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(The Equipe) has the std one which is 34cms (it's a Power Box)......" My Mistral brochures indicate that the Powerbox began to be used during 1993. I have a brochure which says that the fin size for a 1995 Equipe 2 XR/CHS is 38cm."I use a 35 cm US box carbon fin in my 1986-1988 Mistral Equipe LCS XR which is the biggest US box fin I have and I am running 8.5 -9.5 m2 sails and the original sails for this board were 6.5 m2. For the later Equipe and One Design etc the sails were around 7.5m2 so it makes sense that the fin size should now for todays competition and raceboard format be bigger since sails are bigger now than originally. The daggerboard size is also important.
I believe Duff12 is using a 9.5m2 RSX NP sail so a 42 cm fin is not surprising then over the original fin he replaced it with.