shows the "organ at the main altar of the church." The pictures I could find of the altar area of the cathedral show an organ, but it is an instrument installed in a gallery, not like the one pictured. I was able to find information that Gerhard Grenzing did restoration work on the instrument, but the site doesn't include a date, and the instrument restored is the altar gallery instrument, again not the one shown in this postmark.
In this picture above, on the right side, high up on the wall one can see the organ. Below is Grenzing's image of the instrument.
I also found this image (at a tourism site) of an organ in what seems to be a rear gallery in the cathedral. Again, it does not look like the instrument shown in the postmark.
The stamp shown on the cover is Scott 2365, issued 11 April 1984. As such it was issued just prior to the beginning of the conference referenced in the postmark. The stamp shows da Vinci's Study of Man, and the stamp is called, "Man and the Biosphere" in the Scott catalog. The relationship between the stamp subject and the postmark is thus murky at best, and perhaps doesn't actually exist in any meaningful way.
This cover below shows the same postmark, but also includes an image of the gallery organ shown above. One is curious then if the organ in the postmark is really one in the cathedral, or merely emblematic of church organs in general.
I am thankful to my friend Mark Jameson in Reading, GB for the gift of this cover and the enigma of the organ in its postmark.