Went to Library of Congress Newspaper Archives and searched "Eureka Baseball", years 1875-1885. There are seven articles on the Eureka Baseball Club, all from a Sacramento paper, all in 1883:

Here's one:
http://chroniclingamerica.loc.gov/lc...arRange&page=1

Strange there were no articles pre-1883, but Dan's right ca. 1880 is probably as close as possible. It's a great looking image. Hope that helps, enjoy the rest of National Love Your Pet Day.