Four space symposia held in the Netherlands

From a fellow collector, I received several so-called commemorative cards and a postcard relating to the activities of the Hermann Oberth-Gesellschaft, particularly regarding four spaceflight symposia organized in our country by the Netherlands Section. The cards are from 1964, 1966, 1967, and 1969. Wikipedia explained what the Gesellschaft entailed: “An association whose objective is to promote knowledge regarding developments following the first trials of rockets for non-military applications”. The Gesellschaft itself also developed and built rockets that could reach an altitude of 150 kilometers. In 1964, special cards were produced on the occasion of the Space Symposium in Oldenzaal on October 10 and 11. A special postmark was assigned to that symposium. This was announced by means of a Service Order from the PTT: Service Order No. H.504 of September 17, 1964: Temporary placement of mailbox and use of special stamp. 1. On the occasion of the event mentioned below, a temporary mailbox will be placed at a space symposium to be held by the Netherlands branch of the Hermann Oberth-Gesellschaft on October 10 and 11 at Hotel “De Gouden Leeuw” in Oldenzaal. 2. Items delivered to the temporary mailbox will be marked with an impression of a special stamp at the Oldenzaal Post and Telegraph Office. The cards featured special vignettes with images of rockets and impressions of (secondary) stamps with the text: transported by rocket. In addition to the impression of the official special stamp on the stamps, an impression of the Oldenzaal date stamp with the date October 10, 1964, was applied. This second impression was mandatory because the special postmark included a time period, not a fixed date. In 1966, the second symposium took place, again in Oldenzaal, and a special postmark was also made available for this symposium: OLDENZAAL 2rd Space Symposium 1966 Service Order No. H.205 of April 15, 1966: Temporary placement of mailbox and commemorative stamp. 1. On April 23 and 24, the 2nd Space Symposium will be held at Hotel “De Ster” in Oldenzaal by the Netherlands branch of the Hermann Oberthgesellschaft. 2. On this occasion, a temporary mailbox will be placed in the said hotel. Items posted in this mailbox will be provided at the Oldenzaal post office with an impression of a special stamp as well as an impression of the date stamp of that office. These cards will also feature a special vignette with an image of a rocket and a stamp impression with the text: transported by rocket. The third symposium took place in 1967, and once again it was Oldenzaal where it happened. This time not in hotel 'De Gouden Leeuw' or 'De Ster', but in the Oldenzaal Town Hall: OLDENZAAL 3rd Space Symposium 1967 Service Order No. H.301 of June 1, 1967: Temporary placement of mailbox and use of special stamp. 1. On June 9 and 10, the 3rd Space Symposium will be held in Oldenzaal by the Netherlands Branch of the Hermann Oberth-Gesellschaft. 2. On this occasion, a temporary mailbox will be placed in the Town Hall in Oldenzaal. Items posted in this mailbox will bear the imprint of a special stamp as well as an imprint of a date stamp from Oldenzaal. A loose block with the special vignettes (see image) was enclosed with one of the envelopes. BREDA 4th Space Symposium 1969 Finally, a postcard that was available on the occasion of the fourth symposium. The card with the secondary stamp transported by HOG 9 rocket. This time, the symposium was not held in Oldenzaal, but at the Royal Military Academy in Breda: Service Order No. H.656 of October 30, 1969: Use special postmark. On the occasion of the 4th Spaceflight Symposium of the Hermann Oberth-Gesellschaft Netherlands branch, a temporary mailbox will be placed at the KMA in Breda on November 14 and 15. Items posted therein will be marked at the Breda post office with a special postmark as well as a Breda date stamp. No annotation is required for these items. Also included is a stamp impression: transported by rocket. I have not been able to ascertain whether the depicted maps were indeed transported by rocket, nor even where these rockets were launched or where they ended up after the flight. Do you know?