Does anyone know the answer to this?

Did the US always use Mom and Mommy for Mother?
If not when did it change from Mum and Mummy?

I have a early 70's imprint of Heinlein's The Star Beast (originally written in 1954) and that has the word Mum not Mom. Is this an "english" version???? I don't think so as the audio book I have also uses Mum non Mom.

Russ