1943 NDH, Labor service, two sided printings

I got some nice pieces where I first thought that someone has wrongly cut them off. But then I realized that I was looking at the back side...

Back side

Front side

I am not sure, but according to the statement made by Mr. Ercegović, who wrote that he never saw proofs printed on both sides, I believe that they should be very rare.

 Update:  Thanks to Mr. Hughes and Mr. Bokić I learned that these are not as rare as I thought.