Belgian Olympic With Overprint

Why was this overprinted.
Has it anything to do with the first world war.

thanks guys

SC#140 1921
Scotts describes these as 20c on 5c+5c. So the original stamp was a semipostal, the extra 5c probably going to support the Olympics. There are three stamps in this set all surcharged to 20c. They've simply changed it to a regular postage stamp. Maybe they were in short supply of stamps?
The war ended two years before, perhaps they had other things to deal with at the time.
That's my guess anyway


edit- Looking at this stamp (and the others) in the semi post section of the catalogue, the surtax benefited wounded soldiers.

Thanks James.
always good to know the reason.

Indeed, originally semipostal. And yes, the first greek stamps issued for the Olympic Games were intended to partially support the Olympic Game, part of the income entering in a fond of the olympic comitee. But in this case originally the +5 c was going to support the war invalids. The idea wasn`t really well regarded by the public (as in the case of the first Olympic issue, although finally they were all sold, so at the 10th anniversary of the first Olympic Games the Greek Post issued a commemorative set in 1906 ), so Belgian post overprinted this set.
-edit: as Jamesw edited, sorry, I observed it late. Still visible under the red ink the french inscription: `Pour les mutiles`. Nice scan.

; ) good info there Wadmalatz.
i Have to admit it is not a scan. It is a photo I took.