Watson/Hacker exposure meter

Help!! I just acquired a used exposure meter, and am puzzled as to how
to correctly use it. The meter was made in England, and is labeled
William J. Hacker & Co., West Caldwell, NJ, and is a Watson...

It has a large analog meter face, with stepped sensitivity control, and
a slide rule type exposure calculator. The puzzling part is the section
of the scale that must be set underneath the paper/film speed mantissa.
It reads "setting factor", and is incrementally divided 1-12. What is
this setting factor, and how is it determined? Does anyone have any
experience with this meter, and even better yet, have a manual that
could be copied? I have run every search engine I could find, and
nothing at all turns up on this meter. Any help is appreciated.

Patrick