How do I get the Pope to recognize my grandparents’ 50th wedding anniversary with a papal blessing sheepskin?

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Call your local parish and get the number for the chancery (the main office of the Bishop of diocese). The folks at your local chancery will be happy to help you with this request.
Judy Grant, the ever capable and friendly parish administrator at St. Anthony’s Parish in Cody WY (where I’m based while I celebrate Masses in Yellowstone National Park in the summer), suggests writing directly to the office of the Bishop of Rome. Here’s the Pope’s mailing address:

His Holiness, Pope Benedict XVI
Apostolic Palace
00120 Vatican City
email: benedictxvi@vatican.va
The Pontifical North American College in Rome has a Bishop’s office for American Visitors to Rome. I imagine they would be able to help also. Their address in Rome is Via dell’Umilta, 30.