Help to identify Poland item

That's a "stamp" from a pre-stamped envelope - we have a thread for what Rod likes to call "mutilated postal stationery" which contains many similar examples, all cutouts from pre-stamped envelopes and postcards and aerograms and so on.

According to my Michel Eastern Europe postal stationery catalogue, your example was issued on April 15, 1981.

Ryan

This image is from a listing on Delcampe:

There's a small item of interest to add to Steve's cover. The top left corner indicates that the selling price for the envelope was 2.90 Zł - this particular envelope came into service at a time when the price was lowered, and there is also a version that shows a selling price of 3.00 Zł.

Ryan

Unusual stamp - 2.8 million made but fairly rare (except on eBay) & valuable.
Some older books mention a wavy line watermark in the genuine but recent catalogs do not (Fischer, Michel, Scott, Yvert).
The forgeries are abundant and most are excellent.
Really the best way to tell is with perforated examples as these were I believe sheet (harrow) perfed so the stamps are all of equal shape and size - example below.
Genuines are typographed, forgeries are litho.


This doesn't help with yours but I think it is a valuable genuine trimmed to look like the more valuable imperf.
I have seen examples of people doing that just for fillers.
In the 70's I got a nice France collection where someone had trimmed the perfs off dozens of early stamps worth $1000's to use as fillers