The "Stunt" version was made in larger quantities and were "printed" with no letter enclosed so the prop was kept lightweight for the scene when the envelopes were blasting out of the fire-place. While made in greater quantities, each envelope is still individual because of the random "veining" throughout the envelope. The Hogwarts coat of arms at back is a "printed" wax seal, to keep it light-weight. Here is a link to see the prop invitations in the movie.
The item "Harry Potter (2001) Hogwarts Invitation Envelope (Screen Used) Stunt Movie Prop" is in sale since Tuesday, December 3, 2019. This item is in the category "Entertainment Memorabilia\Movie Memorabilia\Props\Originals". The seller is "collectionturkey" and is located in Los Angeles, CA. This item can be shipped worldwide.