Here we go again, with blanks drawn in my effort to identify one of the "over 400" overprints of Bohemia and Moravia stamps made at some stage before the Czech government-in-exile returned to Prague on 10 May 1945. It is this one, from a set of 21 which I have (so not very expensive):
Of the couple of thousand Czechoslovak locals offered on Delcampe, this overprint is strangely absent. A Google image search is less than helpful: their "best guess" is that this is "a postage stamp".
I have other overprints, from Domazlice, Radcany and Rumburk - but where did this one come from?