Not being familiar with them, this posting prompted me to look into the story behind these overprint stamps.
After WWI the first Czechoslovak Republic was formed (it lasted from 1918 - 1938). The new country had huge quantities of leftover stamps from Austria and Hungary so they decided to overprint them and sell them at a 50% premium over face value with the surtax monies going to public charities. They were only sold at the philatelic window at the Prague Post Office.
Here's my source: https://www.stamp-collecting-world...._1919co.html