This occurred for years using a high-quality printing press - the German Goebel photogravure reel-fed rotary introduced late 1968 in the Casa de Moneda in Buenos Aires.
I noticed it for the first time seeing the semi-permanent definitives of 1977:
viewtopic.php?f=139&t=1401
Under the title "New way of photogravure printing since 1976 for definitives?" I published one of my first postings in the Argentina Foro on March 27, 2009 being a fresh collector of Argentina since February 2009!
I had th eidea it had something to do with the method of printing [photogravure] but after quite some time - having studied the types of paper better - i cam e to the conclusion it had nothing to do with the printing!
and in detail:
The 20pesos is still in the tradtional photogravure! That is this printing ....
The 30pesos and 50pesos have the irregular screen ...
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It was the choice of paper! And it had been a deliberate choice!!!! But why???????
to be continued ...