After I WW there were made many private overprints (speculative reasons).
Here are a few examples- PDF file, page 6, under 4th paragraph:
4. Csalások, csinálmányok és hamisitások. - cheat, made, forged.
(under this group there are presented overprints made without the knowledge of the postal authority, to deceive or for their own pleasure.
No philatelic material!)
On
ebay occasionally you can find such overprints, I remember for example the last one presented here (Nagyszeben).
This must be one such overprint.
http://www.mafitt.hu/org/hagyomanyo...201002ny.pdf