UKM application- mother British through marriage?

Hello- has anyone successfully applied through double descent when their father and mother were married before 1948?
My situation is that my father was American born to British citizens (born in England). He married my mother in 1940. I contacted a UK immigration firm and they claim I am British by descent because although I can't inherit my Dad's citizenship, my Mom became a citizen through marriage and therefore can pass it on to me (born in the 60s)
Has anyone here had a similar situation?

There are grounds, as your parents were married before 1948.

To the best of my knowledge, there has never an automatic jus matrimonii route to CUKC/British citizenship since at least 1948.

But, as your parents married before 1948, it seems that your mother would have automatically acquired British subject status by marriage to a British subject (your father)(see Page 8 of the Historical background information on nationality and Section 3.1.3 of the British Nationality Summary) and then have become a CUKC (Citizen of the United Kingdom and Colonies) in 1948 (Section 12(5) of the British Nationality Act 1948 as enacted). She would have acquired Right of Abode in the UK under Section 2(2) of the Immigration Act 1971.

However, before 1983, women could not pass on their citizenship to their offspring. In 1983, your mother would have been classified as a British citizen by descent (Section 14(1)(b)(iii) of the British Nationality Act 1981).

You might be able to register using Form UKM praying in aid the Romein judgment. On the positive side, the fees for Form UKM are only £80, so you can give it a whirl.

Be aware that any children already born to you will not be able to acquire British nationality by descent automatically.