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I once heard that they were issued for the Croation government in exile (?)
After the foundation of Yugoslavia from Serbia, Montenegro, Macedonia, Croatia, Bosnia and Slovenia, some Croats saw the kingdom (with the former Serbian royal family as monarchs) as far too Serbian dominated, and campaigned for an Independent State of Croatia - NDH. These dissidents were predominantly right-wing, and culminated in the rise of the pro-Fascist Ustase Party under the leadership of Ante Pavelic.
When the Axis powers overran Yugoslavia in 1941, they agreed to the establishment of an NDH (also with hegemony over most of Bosnia) as a Fascist puppet regime under Pavelic. At the end of the war Pavelic and some of the other Ustase leadership escaped the Allies and went (at least initially) to Argentina, where they declared themselves to be the NDH in exile.
In 1951 they set up a Philatelic Information Service operating out of Argentina and began issues of stamps. This group was 'official' in that they had Pavelic with them. As Pavelic moved on, to Spain and Australia, possibly other places too, he took some supporters with him and left others behind; also there were groups of supporters in the USA. So it became increasingly difficult to say where stamps were coming from and how 'official' they were.
Also some of these groups began issuing stamps, without claiming to be the NDH. The 1950s issues showing Cardinal Stepanic are a good example.
All these issues were produced in comparaatively small numbers. But two issues were clearly issued in much bigger numbers: they are the triangular Birds/Flowers sets and the 1949 UPU set. Although they bore the NDH imprimature, the 'official' NDH vehemently denied having any responsibility for them.
It is highly likely that whoever (with or without the prior knowledge/approval of the NDH) produced these issues - went on to pocket the money for themselves!