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

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'