CREATE TABLE table_17690 ("Quattroporte IV" CLOB, "Units Produced" FLOAT, "Production Period" CLOB, "Engine Capacity" CLOB, "Power" CLOB, "Max Speed" CLOB)

When were 668 units produced?
SELECT "Production Period" FROM table_17690 WHERE "Units Produced" = '668'
SELECT "Production Period" FROM "table_17690" WHERE "Units Produced" = '668'