Double descent if grandmother was a UK citizen but mother was born in USA before 1948?

HI All - I have been doing some research and even starting some paperwork to apply for British citizenship by double descent but I'm concerned I may have run in to a snag. Details:

Grandmother was born in Wales in 1930. Met a US soldier during WWII and moved to the states.
Mother was born in USA in 1946.

I was born in the US in 1966.

Grandmother and mother are both deceased. Mother never became UK citizen

I am concerned that since my mother was born before the British Nationality Act of 1948 that citizenship may not have been available to her and therefore not to me either.

Can someone set me straight? I am hoping I've misinterpreted the rules.

Thanks!