The Polish Philatelic Federation in the United Kingdom issued 2 souvenir sheets in dark blue and dark brown to commemorate the «Polphilex 66 – Polish Millennium (of Christianity)» exhibition.  The sheets were designed by Jerzy Kwiatkowski and engraved by Czeslaw Slania. 

The sheets have inscriptions of 25/Polska/GR which can mislead people into thinking these are official Polish stamps. As well for the perforate as for imperforate sheets, proofs printed off the plate are known (1st stage of printing) as tete-beche pairs.  Also proofs of the stamps (perforated 11 1/4:11 1/2) in various colours produced on demand  exist. 

Polphilex 66.  Souvenir sheets, mint and cancelled.

At the left:

  • As a favour to philatelists, a perforated version 12¼, was also done by Czeslaw Slania.  When he finished the engraved plate, he printed examples in several colours perforated 11½ in a frame 51.5 x 27.1 mm.  An example is shown to the left in grey colour. 
source: Close-up No. 3/2003, pg. 8.
At the right image to see details of the label.