I was emailing the Boutique in Geneva about this one.
And they very kindly sent me the same article.
The fourth year ie the leap year. I can only assume that Bissextile is French for the fourth or something similar??
For the remaining three other years you will have 1FE 2FE & 3FE
The FE is simply the first two letters of February.
Every other month apart from February will have the first three letters, such as MAR for March.
I hope that this helps you.