Info on 317a Oxygen Regulator

Hi,

I have a 317a Chest Mounted Oxygen Regulator (L. Adams manufacture) and am trying to find any literature or instructions on use. It has a selector switch between 100% and mix, which I understand the use of, but there is a small two position selector dial which I am unsure on.
If there is anyone who has used them in service or perhaps worked as a Safety Services Specialist I would appreciate the comment.

I can supply a picture if it would assist.

Thanks in advance.