Revocation of cancellation of registration: one-time opportunity to apply subject to filing outstanding returns within specified timelines. Clarification explains that where registration was cancelled for failure to furnish returns, an application for revocation of cancellation requires prior filing of all returns due up to the cancellation date; if revocation is ordered, returns for the period from cancellation to revocation must be filed within thirty days. For retrospective cancellations, a proviso permits filing for revocation provided all returns from the effective date of cancellation until revocation are submitted within thirty days of the revocation order, pursuant to a Removal of Difficulty Order and amendments to Rule 23(1).