Barry Goyette has put together an index of the Register of Separate Property of Married Women and Sole Traders 1856-1913. While I could tell you about the wonders of California's Sole Traders law, why redo what Judy G. Russell, The Legal Genealogist, does much better. In two posts published the week of her recent seminar at SMCGS she explained the law and gave examples of what the records might tell you.
Reprise: California’s sole traders - Judy G. Russell, The Legal Genealogist October 30, 2014
Reprise: Separate property, separate debts - Judy G. Russell, The Legal Genealogist October 31, 2014
You can the index here:
Index to San Mateo County Sole Traders Register - Barry Goyette 2014
And I am very happy to tell you that you do not need to order these records from the county!!!! They are available on FamilySearch. Be sure to start with a page number from the index to search through these browseable records.