In his article titled California Mission Brands, author Bill Reynolds writes
California cattle descended from stock brought to the New World by early Spanish explorers, and it may have been Cortez who brought the first branding irons to this continent. In the late 1700s and early 1800s, mission cattle were branded as a matter of course. Over the years, research on the actual brands used by the missions has unearthed images of the brands used by each, and while many of them have specific histories some continue to have only obscure backgrounds. Every mission in the chain from San Diego to Sonoma had its own cattle herd, some of which were quite large. Mission San Luis Rey had more than 27,000 head. Smaller missions tended approximately 2,500 head. By 1828, the 21 missions controlled or owned almost 200,000 head.
To see the designs for the 21 brands, see the PDF posted on his website