According to https://en.wikipedia.org/wiki/Event_cover https://en.wikipedia.org/wiki/Event_cover they are "decorated, stamped and canceled commemorative envelop

According to https://en.wikipedia.org/wiki/Event_cover https://en.wikipedia.org/wiki/Event_cover they are "decorated, stamped and canceled commemorative envelopes" Essentially they are envelopes with stamps and a marking over the stamps that prevents the envelope and stamps to be used. https://en.wikipedia.org/wiki/Cancellation_(mail) https://en.wikipedia.org/wiki/Cancellation_(mail)