Here's my guess.
F= Featherweight.
The Superlights are models in their own right within the 400/410 range. Here's the page from BAR.
Two models. Deluxe with what looks like the stocks made from Poplar, and classic. I wouldn't be upset if my stock was in Walnut.
EDIT: The Pellpax site clears it up a bit. There are three stocks available in the Superlight series.
Deluxe- Shiny two tone finish, made from Poplar.
Hunter- Green matte finish, Poplar,
Traditional- Brown wood (!), looks like Beech but could be Poplar with a finish applied ?