This is the block number 3. Traveled on the envelope.
The cancellation is to the event.
Envelopes are similar in all auction catalogs from $ 400-500 to rise.
Well would check that the package has not been pasted after.
If there is behind the cancellation of Vienna there should no problem.
I consider it a FDC.
I could not quite make out the special cancel date but it does appear to be June 23, 1935 so that is the release date unless that 23 is a 25 then it would not be the release date but two days into the exhibition.
As to first day cover status, or general cover status, note that the sheet is not tied to the cover by either postmark and that could be problematical for a FDC collector and cover collector as well.
Any idea of it's worth.