Set-Off German 1923 Air Mail Stamp - Well Defined

Not understanding the question.

I will say that the term "set-off" is an awkward term that seems invented by some that think that others are too obtuse to understand the difference between a printing method and ink on the back of a stamp from contact with another wet stamp. Context is everything and if someone is asking about ink on the back of their stamp or someone else is told to look for ink on the back of a stamp to prove or disprove something it seems a folly to have to dumb things down by using a "special" word. Why can't "offset" have multiple meanings? Most audience members will get it.

Edit to add:

The PF uses "offset"

While we are on the subject of setoff, I have seen lots of examples in the EFO section of ebay that are not setoff, but rather stamps that were partly stuck down on other stamps and when separated took some of the underlying stamp's.ink.

I'm with rogdcam on this.

Perhaps it's time agree to disagree as nothing will be settled, and instead return to the original intent of the thread showing examples of German stamps which fit the OP's request.

It is a theme that is nearly one year old, perhaps somebody still likes to know what the reason of the coloured back is.

The stamp has a indirectly printed back on the gummed side - that happens, when the printing machine runs out of paper and now the transport roller is inked a little bit. The next sheet picks up the some of this ink with the backside.

In the 1920 - 1923 the stamps lost Their postal value very fast - so old stamps have been overprinted to fill the needed face value. This overprints appear similar on the backside sometimes for the same reason,

In germany this is called "abklatsch" - you might like to use this information to dig out more pictures -
It is a random effect - so it doesn´t appear in the catalog but still interesting and collectible. I add a picture of such an overprint on a offical stamp from 1922