On San Diego County Route S-2 in Vallecito Stage Station. 37349 County Rd S2 (P.M. 34.7) Shelter Valley, 3.7 mi NW of Agua Caliente Springs
The semi-weekly horseback mail delivery between San Diego and Yuma, Arizona made Vallecito a regular stop along its route and In the fall of 1857, the US received its first overland Atlantic to Pacific mail service when James E. Birch's San Diego-San Antonio mail began operation.
http://www.sdcounty.ca.gov/parks/Camping/vallecito.html