This envelop specifies that it is "philatelic correspondence," probably to let postal workers know to treat it carefully. Probably it is merely the postmark which the sender wanted to conserve through the postal journey. The stamp is Scott 2664, issued 13 June 1998, for a National Music Festival. It was the Europa theme for that year. As such it is at least tangentially related to the postmark. The instrument in St Jean de Moirans was installed in 1978 by Xavier Silbermann. The same builder added stops to the pedal division in 1981 which completed the instrument. It is small, a mere 713 pipes. This site gives the specification and a few more photographs. Finally this site also discusses this little instrument. This one notes a need for restoration work on the organ, and a fund that has been set up for the project. Without more information it would seem the postmark is simply to promote a series of concerts on the organ scheduled for 18 June 1998. Moirans, France is in the former Rhone-Alpes department of France, near the Swiss and Italian borders. At a little less than 8000 residents, the population is similar to the town where I currently live!
I recently came across this envelop as I was doing some more cataloging of my collection. It shows a pretty view of the church in the cachet. The postmark shows the organ and some pipes, though it is faint.
