Even that I don't like countries that made genocide around, I must to mention this issue! Artist made it well, and we can say that it is really one "masters work" when is talking of engravings stamp design! It is made with full emotion, and You have feeling to be there, on the street of old city Zagreb!
I will give just some information. This issue come out for III. Phila Exhibition held in Zagreb - Croatia (at the time: Independent Country of Croatia, under German Occupied Forces, some say "puppy state"). Graver was K. Seizinger, printed in Wien - Austria, existed perforated and imperforated, overprinted, with "hidden" graver "S" mark, and without graver mark.
Very hardly, that You will find it somewhere around in perfect condition, because total quantity printed run around 118 000 m/s, take in account WWII period, Communist period after WWII, and You will see that most of it have some fault, or are totally damaged in some way!
What catalog say regarding prices? Keep an eye, if some samples will be available for exchange - take it right away! Michel value it this way:
simple option = 18.00 euro,
with graver mark "S" = 55.00 euro,
imperforated = 350.00 euro.
Which option You have in Your collection?
Just don't mix it (graver mark "S") with stamp and m/s - because graver mark on stamp are inside "window" and on m/s is on the plate of near door on building!
Because "problematic" paper and glue (there is two type of paper, one "hard" and one "Japaneses" more tin), many samples are damaged by humidity, after it - collectors used cancel to "repair" damage - that resulted in some "fake" cancel applied on some examples - existed some very good Phila Societies to find more information, like Yahoo groups on NDH, APS and some other.
Again - search for it, and take it at the spot!