PRZEDBÓRZ, Fi 3a–6a stamp series, PROOF PRINTS
Fi 3A ** pink background, background offset
Fi 4A P2 ** green background, background misalignment
Fi 5A P2 ** pink background
Fi 6A P2 ** green background
unperforated stamps, without backing paper
MIKSTEIN guarantee [stamps on the back of each stamp in the series]
Reference: Fischer, "Catalog of Polish Postage Stamps (and Those Related to Poland), Volume II [2022 edition], p. 65
