Final exam form

Dear all, I have registered for ca final on 4th Aug. 2015.So obviously I will get the Final Reg. Letter after 15 or 30 days.But my final attempt is in Nov. 2015.ICAI staff told me to register for final examination & don't wait for reg. letter.Now while filling the exam form online, Old Article Reg. No. is asked in the form.So my query is which no. should i write in this field??????Pls reply. It's urgent.