These two stamps are a set of four issued by Belgium in 2000 for promotion of tourism. All four stamps depict a rendering of the exterior of the church building, plus a view of the organ. The first (Scott 1822) stamp features the organ at the Norbertine Abbey in Grimbergen. The second (Scott #1823) is that of La Collegiale Sainte-Waudru. Here is a nice article on the builder, Delmotte.