Looks like uncommon from Poland

It seems that zloty with socialist stars on eagle of year 1967 is a bit tricky: 

source: https://www.muenzauktion.info/auction/item.php?id=3173009

not very commonly seen in collections.

The 1967 is the rarest year, mintage is only a little above 1 milion pieces. Total mintage of all years was over 900 millions...

N#2065

This may explain, why it is seldom seen and in constant demand for collections. 

But the mintage may be not the explanation: as wrote Uzdenikov in his book dedicated to mintage, mintage has nothing to do with rarity: there are common coins with small, sometimes ridiculously small mintage and there are rarities with mintage in millions.  Most probably the last is our case: the given mintage is not for the year on the coin. 

I don't think this is a very rare coin… at the moment, I found 28 pieces from year 1967 avaliable only on one auction platform, here in Poland. 

It is very simple. When a collector feels that the price for coin is underestimated it is very rare seen on sale. But when a price just right, it is in constant preposition.  

For example the coin 

(Source: https://t.me/s/enjoyyourcollection?q=Lot+451 )

is essentially in less quantities on preposition, which means, that collectors believe that the estimation is too low and do not in hurry to get rid of their treasures. 

Though most probably the definitions are necessary to understand properly, what does it mean, “the rarest year” and “don't think this is a very rare coin.”

Maybe the 1967 of 1 zloty is the rarest, though it looks that the 1968

source: https://t.me/s/enjoyyourcollection?q=Lot+6171

 is important as well.