Claim based on mother British by marriage before 1949

Hello,
BNA 1948 section 12(5) sets out that such a (British Subject) woman 'shall herself become' a CUKC on Jan 1st 1949 if her husband so becomes. This appears to be automatic and to require no registration of any sort. Can this be so? Claimant still to obtain all/other necessary documents.
I assume this would be a Section 4C claim via form UKM, with mother classed as British 'other than by descent' and able to pass her nationality to claimant? Father had UK-born parents; mother South African; claimant born in RSA in March 1949.
Thank you.