In the September 2019 edition of “Stamp Collector” an article was published on the Polish government-in-exile stamps. The article titled “Poland’s exile issues” tells the story behind both the official exile stamps, produced by the London exile government, and the stamps as produced within wartime Poland for the underground mail.
Both the 1941 and 1943 series are shown including the 1944 overprint for the battle of Monte Cassino. Also shown are the lookalikes from wartime Poland and the 1945 issue commemorating the Warsaw uprising.
Here are the pages shown in a much smaller size:
More information on Stamp Collector magazine can be found on the All About Stamps website. Do buy it, the rest of this edition has some other fascinating articles on Second World War related philatelic subjects.