16:04 Feb 1 

"une autre rainure est formée plus en avant dans la surface de l’alésage."
i.e. omitting the "le" before "plus" = further forward.
With the "le" one gets "the furthest forward" i.e. at the very end, as in my original answer, which sounds a bit odd to me.
Anyway, I don't think it's absolutely crucial to the description, and there must be an annexed drawing to confirm this groove's exact position.