Stamp day 2013 with engraver signature (!!!) on insured registered cover from Serbia
I received this insured registered cover in February with block of four stamps Stamp day 2013, issued on 25.10.2013. There's a specific philatelic specialty in philately of Yugoslavia and Serbia - hidden signature of engraver with long tradition over many decades.
On this block you can see hidden signature on bottom right stamp - NS. Location of signature is never revealed and you never know which edition will have it, and sometimes is really well hidden. And for your information, stamps with engraver signature cost much more on philatelic market!