Loveletters from Scandinavia

I scored this paper artefact on the ever so powerfull Eb*y. All the way from Scandinavia this is a real unused stamp depicting a, you guessed it, Bubbla. Supposedly this would date back to the late forties and if you look real close you can barely make out it's split window, which could indeed date it back to as early as pre-53. If anyone knows anything about these, do tell.