I am a newbie with flash, but try to use my wifes Cullmann SL 28.
I can't find any user manual, either in the house or on the net. So I wonder if anyone here can help, or have the manual.

The black knob at the center of the back (see picture) changes the asa value at the top, moves the aperture scale and change the numbers placed just under it (knob).

I think I understand how to interpret if the white knob is set to the red dot, but I'm confused as to how to read the values when set to the grey dot.

(I'd also like to know something about the M. Would that be for giving the control to the camera?)