Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_67999 ( "Year" real, "Men's singles" text, "Women's singles" text, "Men's doubles" text, "Women's doubles" text, "Mixed doubles" text )

### Question ###
Name the mixed doubles for year before 1991 and men's doubles of david jepson buck ryan

### Accurate SQL ###

SELECT "Mixed doubles" FROM table_67999 WHERE "Year" < '1991' AND "Men's doubles" = 'david jepson buck ryan'