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I also noticed the ciruclar overprint instead of the square, so not sure if its genuine..
Also what is surprising here is 2 overprinted to 3, 6 overprinted to 5 and 20 overprinted to 2 - not sure why they did it this way, its neither inflation nor deflation! ;-)
Circular block out, is consistant on that later issue.
I am not a specialist with Yugoslavia, I would suggest the surcharges were to address postal rates at that time. Perhaps to make use of remainder stocks.