APO Ronar 300/9 - what are these lever and scale good for?

Hi all,
I've finally bought an APO Ronar 300/9 I wrote about recently. It looks O.K., with respect to it's 20 yrs. age, seems to have single layer coating, simply - I think I will love it.
What I don't know are two things: there is another (red) scale from 2 to 30 mm on the opposite side of the aperture ring, and a lever just above the ring (it can move about 100 degree or so, but seems to rotate with nothing inside). See both below.
http://www.hibrno.cz/files/ApoRonar06n4.gif
Can anybody explain what are they useful for?
Thanks, Michael

1. sorry for such a big image - I know it could be half a size as well...
2. it may not be so obvious from the image that it is a process lens without shutter (but with apertures upto 260! wow!) Michael

The lever opens a slot for the insertion of waterhouse stops, and the mm scale is just that, the size of the aperture in mm. Both lever and scale are useful for someone using the lens for its orignal purpose in the graphic arts, but neither are of use to the LF photographer.

Some people stick filters in the Waterhouse slots.

Yes, I can see it - thanks to all.
I have noticed that the number doubles every two stops - and the explanation is so clear...
And regarding the waterhouse stops: probably I will never use them :-) I'll try to stick the lever in "closed" position or even remove it.
Yours, Michael