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_17690 ( "Quattroporte IV" text, "Units Produced" real, "Production Period" text, "Engine Capacity" text, "Power" text, "Max Speed" text )

### Question ###
When were 668 units produced?

### Accurate SQL ###

SELECT "Production Period" FROM table_17690 WHERE "Units Produced" = '668'