1945 - Pro BENI Department on their centenary (100th anniv.)
Compl. set:

In below stamp the text mean:
Recargo del 20% = 20% surcharge
Ley de 1º Febrero de 1945 = Law of 1 february 1945
Pro desayuno y ropero escolar = Pro breakfast and school wardrobe
It must be a charity stamp to raise funds