Mary's grandson is pretty active online on his family's history. Looks like we may never find out about old Sonny. 

His response:

Thank you for the interesting postcard, apparently from shortly before my grandmother came out to Washington. Unfortunately I don't know anything about the writer, or Sonny, or even any connection to the places mentioned. My grandmother's family had immigrated to Iowa from Switzerland before moving to Almira, Washington, and my mother had contact with other members of the extended family, but all of that generation are long passed, and I have no information on their descendants.