I think that there are several members on SCF who collect Esperanto material. There are 2 lots on eBay listed as cinderellas with quite some labels. These are NOT my lots, I just did see them and they are difficult to locate on eBay. https://www.ebay.com/itm/224548467175
The writer of the card, one Erich Bergmann, says of the congress: "The 4th International Esperantist Congress was truly very beautiful. I had the honor of meeting Dr. Zamenhof [the inventor of Esperanto]."