Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE table_1245350_1 (production_period VARCHAR, units_produced VARCHAR)
### Question:
When were 668 units produced?
### Answer:
SELECT production_period FROM table_1245350_1 WHERE units_produced = 668