i'll be corrected and tarred and feathered if i'm wrong, i believe aikikai registration number is received at the dan level.
You receive the Aikikai registration number when you register with the Aikikai. It has no reference to rank. I know two people who have the number and are not ranked. The students at one of my dojos all get their Aikikai number at the time they test for 5th kyu just because that is the convenient time.