6. Upon application made in that behalf in the prescribed manner, the Minister may, in his discretion, grant, in the prescribed form, a certificate of citizenship of Sri Lanka by descent to a person with respect to whose status as a citizen of Sri Lanka by descent a doubt exists ; and a certificate issued under this section to any person shall be conclusive evidence that that person was a citizen of Sri Lanka by descent on the date thereof, but without prejudice to any evidence that he was such a citizen at an earlier date.