At the time of this picture postcard, the owner of about 99 percent of Santa Catalina Island, William Wrigley, Jr. also owned the Chicago Cubs baseball team. He brought them to the island for spring training. The training ground seen here was between the town of Avalon and the golf course, up Avalon Canyon. The number on the back of the card is A-87596, which, according to the guide maintained by the Curt Teich Postcard Archives, translates to 1921.
