1. Issuing notice under Section 74 without an element of fraud or suppression of facts with an intent to evade tax manifest on records is unsustainable and unjustifiable. The essential onus lies on the authority to prove the fact of evasion of tax, if any.
You can ask for the incriminating material that has caused the issuance of such SCN. Mere payment of tax under wrong head is never an attempt to evade tax, much less fraudulently.
Normally the authorities resort to such backdoor entry to have the advantage of extended time period. But this is highly unacceptable under the law and by the judicial courts as well. You can contest it on merits. Law is well settled on this issue in favour of taxpayers.