(b)When any stamp used for an instrument has been inadvertently rendered useless under Section 19, owing to such instrument having been written in contravention of the provisions of Section 18, the Collector may, on application made within six months after the date of the instrument, or if it is riot dated, within six months after the execution thereof by the person by whom it was first or alone executed, and upon the instrument, if chargeable with duty, being re-stamped with the proper duty, cancel and allow as spoiled the stamp so misused or rendered useless.
