Polis-Tidningen 1/1, 1946
(I saw this in Problem och profiler)
11+1. Add a White Queen and then mate in 1 a) Diagram b) Mirror position
The retro-analytical trick here is quite simple. One just has to observe that the original White Queen never left the 1st rank because the wh. Pawns never moved. Also, no Pawn promoted into a Queen, so that we can only add the original thing. Finally, the Queen and the King could not cross, so that there are only three squares where a White Queen can be legally added: a1, b1 or c1.
Solution: +wQa1 & 1. b3 #
For the mirror position, the same reasonning applies. (The Kings now are on e1 and f4.)
Solution: +wQc1 & 1. d3 #