Registration continuation requirement: allow liberal condonation where Form 12 was filed late with returns to prevent hardship. The amendment requires firms to furnish the Form 12 declaration within the time allowed for filing returns, which could disqualify continuation of registration if declarations were filed with returns after the amendment's effective date. The proviso permits Income-tax Officers to allow late filing where prevented by sufficient cause; the Board directs liberal exercise of that discretion for earlier assessment years to prevent hardship and condone delay where declarations were filed with returns after the effective date.