The 13-cent California State Flag stamp (Scott 1663) features the Bear Flag, which William Todd painted and which was first raised on June 14, 1846, at Sonoma by a group of American settlers revolting against Mexico's rule. The original Bear Flag inspired the current California state flag, adopted in 1911.
The bear image represents the numerous grizzly bears in the state as well as great strength. The words "California Republic" commemorate the American pioneers who settled in the territory. The five-pointed star was taken from the Lone Star of Texas.
The original Bear Flag, held by the Society of California Pioneers, perished in the Great Earthquake and Fire of 1906.
Scott 2005 Specialized Catalogue of U.S. Stamps and Covers
SHG Resources (http://www.shgresources.com/ca/symbols/flag/)