At some time after the war, Belgium reduced postal rates by 10%. Stamps for the new rates were produced by surcharging existing stocks by applying the "-10%" notation. There is a thread about them on SCF somewhere. I understand there are hundreds of varieties of the overprint. In fact, there is a catalog devoted to them, but the name eludes me right now. I have a few of these stamps in my collection.
Regards, Robert
I found a pic of some of them
