@ secret.simon
Thank you for your valuable reply.
My grandfather worked in Her Britannic Majesty's Department for Environment Far East Region. In his job certificate,it indicated as Electrician, Overseer and Artisan mate (various periods). It was not a high position job. I don't believe that he was recruited in the UK and sent abroad. My mother told me that 'her maternal-grandfather' was born in India and moved to singapore. Then he brought her father(my grandfather) to Singapore from India.Then he married my grandmother in 1958.
But my grandfather got citizenship under 6(1)(b), BNA-1948, that says,
he is in Crown Service under His Majesty's government in the United Kingdom.
Also there is a right abode paper on my grandmother's indian passport that says 'Certificate of entitlement to right of abode 2(1)(b)[1981]'.
My mother doesn't know, was she registered or naturalised. They moved from Singapore to India, when she was in 3rd standard or so. She lost her old passport. She doesn't know whether it was Singapore or CUKC. Her relatives took admission for her in Indian school as she was born in India. In her school certificates her birth place is in India. So, she got an Indian Identity.
We only have a declaration paper typed and signed by Grandfather in 1980,UK. It was a sponsorship declaration or so. In that, my mother's name is listed under the section(Name, address and relationship of dependents in India who are being sponsored:). The purpose of stay is typed as: Permanent stay with parent in England. But my mother's age was 19 on those days. I don't know what happened next. Mother told me that she was over 18, so her father couldn't bring her to the UK (I am not sure at this). So she got married in India and stayed here.Her younger siblings moved to the UK.
How can I check if my mother's registration was done or not ? I requested the National Archives, UK for checking my mother's registration. But they don't have such details. Also they don't have details of overseas registration too.
My mother is now an Indian citizen. Maybe before that, she was a Singapore citizen or CUKC. She was born in 1961, in Singapore. I learned from the internet that, during 1959 to 1963, Singapore was partially independent (as a crown colony). Citizens born during that period have a Singaprean version of British Subject passport- British Subject: Citizen of Singapore. The other was British Subject: Citizen of the United Kingdom and Colonies, which was my grandfather's first UK passport. So, what was my mother's status if she has a Singapore version of British passport.
I have been researching and checking in various documents, studies, reports and blogs the last 3 months. I couldn't conclude my mother's status yet.
I have seen some helpful people here. So please help me to sort this out.
Thank you.