Sorry for the late responds Micky and Seb.
The stamps were sometimes used on feldpost letters mostly not.
Like the Dutch stamps they were sold for the benefit for in this case Dutch legion. So i imagine that the numbers means that some of the goes to the funds.
something like the stamp is 12 1/2 worth but they paid 87 1/2 for it ?
This is what i think it is.
Any information about Flemisch in German service (Waffen SS, OT, NSKK, Kriegsmarine, ...) during WWII is welcome.