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_1245350_1 ( production_period VARCHAR, units_produced VARCHAR )

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

### Accurate SQL ###

SELECT production_period FROM table_1245350_1 WHERE units_produced = 668