Mixture Of Czecho-Slovakian And Belgian Philatelic Product

I do not know what to think of this mini sheet.
This is the mini sheet which came out in Oct. 1937 for the philatelic exhibition in Bratislava (now capital of the Slovak Republic).
In 1945 it was overprinted in Brussels at the occasion of the stamp exhibition there and sold in favour of the village Lidice.
Only the Belgian stamp in the middle has the special cancel for the First National Festival Day.
Now my questions: 1. Is this a private product or (semi-)official?
2. Into the collection of which country should I
put it?
3. Is there any value to it?
Thank you!

Good question...the Czechs created it and the Belgians put their mark on it..i would say it belongs in a Cinderella or back of book collection ! The Scott catalog mentions it but does not give it a catalog number !

The Scott catalog mention is under the original sheetlet in the Czechoslovakia section..obviously this sheet was never meant to send anything through the mail !!

Granted, I'm Dutch and not Flemish, but the spelling of the country name at the top looks weird. It's Tsjechoslowakije, or Tjechoslowakije, but I've never seen Tsjezkoslowakije. But like said, it might be a Flemish spelling that I've not seen before.