I don't know how many of these you've got at BtP, and I really don't know if they are of any interest. But if so, here's one of my card collection. I'm not that much into the Polish language, I'm afraid, but to me it looks as if "Procol Harum" is written on the back of the card.
It came with an envelope made for it, with the band name written on in old, great typewriter.
This is like a flexi single, except for the fact that the shape of it is always square, and it is meant to have two functions, a postcard as well as a music single!
(These old-time cards have the same function as all those "happy birthday to you" electronic cards these days ... but the music!!! Oh yes, the music is at times - just like this - wonderful of course, when the complete AWSoP can be played out of the card on a normal turntable, at 45rpm!
The sound quality? Well, could've been better, to say at least ... but still!)
It is made in "cardboard" (thick paper), with a thin touch of plastic on the front side (where the city is on my card). You can see the hole in the middle of the card, just as a normal flexi. The grooves are "printed" into the plastic, and it works!
As I am Polish I want to add few words to Jonny's description.
First of all the handwriting on the card says 'A whiter shade of pale' (Bielszy odcień bieli). Literally translated 'Bielszy odcień bieli' into English would be rather 'A whiter shade of white'. The second line says of course 'Procol Harum' (misspelled). Polish 'ą' vowel is somewhat close to the 'ol' or rather 'on' syllables. As you notice also 'Harum' is misspelled. The city on the card is of course Warsaw (my home city). The card shows Plac Zamkowy (Royal Castle Square).
All the best!