You should be aware that these were forged. After WWI the Germans tried to sabotage the new Polish republic by releasing forged money & stamps into the economy. In the event, by the time the stamps were available, the genuine ones had been de-monetised so the forgeries were released onto the PHILATELIC market to recoup the printing costs.
Both orange & blue sets (for the north & south of the country) were forged, genuine are perf:11.5, forgeries are perf:10.8. There are footnotes in all Polish catalogues about these but collectors don't seem to bother about them. They're not the most exciting of stamps, poorly printed, you can find fly-specks on virtually every copy.
No-one has ever come up with an explanation as to why these particular stamps were forged.
