...this come on end of WWII (1945), as fresh made Yugoslavia, overprint on Montenegro issue (under Italy occupation), set of 11 values,
and additional PORTO issue of two values.
As technology at the time was not so advanced (according today's standard), there are some errors in typing, probably in printing plant, there are some missing latter so worker use "similar" letter in composing this overprint.
Here You can see one error on "A" letter (instead, used "D").
Michel catalog value it well, two sets and one value with error, can reach on market more than 700.00 euros, as this issue are very hard to find on market, reason are 7000/10 000 sets printed.
So keep an eye on this issue, and if You can afford, buy it!