CREATE TABLE table_67999 ("Year" FLOAT, "Men's singles" TEXT, "Women's singles" TEXT, "Men's doubles" TEXT, "Women's doubles" TEXT, "Mixed doubles" TEXT)

Name the mixed doubles for year before 1991 and men's doubles of david jepson buck ryan
SELECT "Mixed doubles" FROM table_67999 WHERE "Year" < '1991' AND "Men's doubles" = 'david jepson buck ryan'
SELECT "Mixed doubles" FROM "table_67999" WHERE "Men's doubles" = 'david jepson buck ryan' AND "Year" < '1991'