There were turbulent times during WWII. In spring 1941 Yugoslavia officially signed Tripartite Pact (originally concluded by Germany, Italy and Japan as a defensive pact, later also signed by Hungary, Romania, Slovakia, Bulgaria), which consequently caused coup d'état and renouncement of this decision (de facto). This did not pleased Axis powers and they invaded to Yugoslavia. Vardar Banovina (which was the name of today's republic of North Macedonia) started to be occupied. Except western parts where Italian army was present, majority of territory (central and eastern parts) was annexed by Bulgaria. And it meant that Bulgarian leva was put into circulation in most parts until 1944. It is not clear to me what exactly happened in 1944 but I found some evidence that already Yugoslavian dinar was also put in place. So this validation most probably could be for use of partisans or to bridge some gap between events. I assume it was not a massive action because examples of them are not visible on the market.
Handstamp says: Democratic Federal Yugoslavia, Finance administration of Macedonia
I have seen many fake handstamps during my years of collecting but this one I bought in auction organised by reliable person Borna Barac, author of the book Banknotes of the States of the Former Yugoslavia 1767 - 2002.