In my opinion the notice is without jurisdiction since registration ousts jurisdiction under Sec 63 due to the following reasons.
1.The section starts with a non obstante clause, meaning whenever the provisions of Sec 73 or 74 applies, the provisions of Sec 63 of the Act cannot be invoked.
2.Tax payers who have obtained registration cannot be directed to submit information and records pertaining to period prior to date of registration and initiate proceedings under Sec 63.
3.There is no authority in Sec 63 for the proper officer to secure tax payers involvement in investigation.
4.The section can be invoked only when such unregistered person has 'FAILED' to obtain registration. As such, once registration is obtained use of best judgment method permitted in case of unregistered persons cannot be applied against registered persons even for a period prior to date of registration.
5.If past failures (for the period prior to registration) are open to proper officer to act upon under Section 63, the provisions of Sec 63 will be retro active (taking steps prospectively of anterior facts)
The best option under such circumstances is to approach the High Court to quash the proceedings because the Section grants the power to the Officer to make a unilateral assessment as per his best judgment. Such judgment would invariably result in a WORST JUDGMENT assessment.
There is no mention in the query as to the contents of the notice and as such replies are based on pure reading of the section.