When notice u/s 61 is invalid

Both are valid......

U/s 61 (3) In case no satisfactory explanation is furnished within a period of thirty days of being informed by the proper officer or such further period as may be permitted by him or where the registered person, after accepting the discrepancies, fails to take the corrective measure in his return for the month in which the discrepancy is accepted, the proper officer may initiate appropriate action including those under Section 65 or Section 66 or Section 67, or proceed to determine the tax and other dues under Section 73 or Section 74.