It is the aerator timer. There is a small circuit board inside the dash and the shaft of the dial comes out of the middle of the board. It has max. min on the dial and works when the switch is in Auto mode. I can never remember if it is maximum aeration or maximum time between aeration! I have seen aeration timers for sale on some boating supply websites that look exactly the same.
I do have the owners manual. Maybe I could scan it. There is a lot of generic info in the manual.

Mine is a '99 519VS.