Here's a pic of an early 1929 mixer panel. This is an illustration from
The Shattered Silents by Alexander Walker. It's an excellent book about the transition to sound in Hollywood. It was first published in 1978 and it's an amazing piece of research by any standards, and especially one from the pre-internet days.
When I have a minute I'll go through a pile of
Radio Craft magazines from the late 20's because I know I've seen an article about mixers. Or I think I have .
Jim