I am pretty certain these are from the soap powder stamps - there are an amazing number in Holland. They come in blue and in yellow and amber, and I would not be surprised if there were other colours as well. Yours is exceptionally large, it must have cost a lot of stamps. ("Zegeltjes")
the technique would be slumping - i.e. heating a plate of flat glas over a mould, and pressing it in shape. The edge then needs to be removed. In the same operation an enamel decor can be added, then fused in the annealing oven.
the capacity to do this would be at Flamingo in Halfweg, or they had it made at Marsberger or Flabeg in Germany. Ming you, these are just suggestions, not fact.