Take a look at this page, which has the Kowel and Krim overprints (and no others), described as propaganda issues:
http://hourmo.eu/15_Stamps_&_Cartoo...paganda.htmlThe Krim overprint is shown on a feldpost stamp. I'll reserve judgment, but I have my guess about these.
As for the underlying stamps being prepared but not issued, that wouldn't
preclude them from being pressed into service with an overprint.