Whilst researching an old developer - 'Panthermic 777 Developer' - I stumbled upon a reference to it in:
"LEICA MANUAL. A Manual for the Amateur and Professional Covering the Field of Photography with the Leica Camera." - Willard D. Morgan & Henry M. Lester, 1935, reprinted 1938
I had not heard of this book but it is a fascinating read and the old ads at the back are truly wonderful.
For those not familiar with it you can obtain it various formats here:
Internet Archive Org
Now I have to find a hard copy version!