Welcome to the forum Dvaranin
Actually, any of the "10%" surcharges you have will have been issued in 1946. This was done to relieve some economic pressure on Belgian citizens recovering after the war.
This was done to alleviate costs of posting mail for customers just after the war. They will be even face value stamp such as 5 Francs reduced to 4.50, 2.50 Francs reduced to 2.25 Francs and so on.
There are a few thousand type of these. Some, instead of having "-10%" will have "4.50" on a 5.00 Franc stamp - in other words, it has already been reduced the 10%.
To be honest, this is a study all on it's own with a lot of Belgian collectors. I also specialize in these. There are also some postal cards with the same type of surcharge.
If you really want to get interested in these, I would suggest you try to find a copy of a book called "Catalogue National - Specialise "-10%" by Frank van Acker. It is long out of print, but, still worth searching for.