Reply to stampfan1980 RE: The 1940 Marseilles Esperanto Congress issue
I see the 1940 Marseilles Esperanto Congress issue comes up on eBay
once in awhile and they are almost always under US$5 except where the seller thinks he is selling the Crown Jewels of China and this is the center piece.
Keep an eye out and you should be able to find one (if you haven't already).
Another easily missed event is the -Israel 1967 52th Universal Congress of Esperanto in Tel Aviv, 2-9 August 1967 label- which was cancelled due to the 6 day war. Label size 40x39mm.
The congress was held instead in Canada which used the SAME design but with the appropriate changes of location & date.
Both of these also come up on eBay
once in awhile, usually at a modest price.
One of the only others I know were canceled was the 1914 French "Castles Of The Loire" issues. I hope the information is useful.
As to more modern label issues, I don't see a lot of new stuff coming out. As a rule the Universal Congresses still issue labels, but I have only sen them on eBay
or Delcampe, as it seems EUSA only has them as they become available. The last UEK label I saw that I saw they carried was the 92a Kongres en Jokohamo Japanio (not listed in their enventory at this time 2020. Later labels can be found on the internnet. I don't know if the UEA carries a stock of old labels or not.