Brett,
Your tiller does have automatic depth control. The handle with two notches I think is either the On/Off mechanism, a two position feature for the depth control. I can see in the photo that there is a cable attached to the forward end of the rod which is attached to (or runs through) the handle and two notches. The other end of the cable attaches to the lever system for the 3pt lift. When the dirt flap at the back of the tiller moves up, it pushes the rod forward, which pulls the cable to actuate a lever on the 3pt lever system. This motion should lift the tiller. And there you have it - automatic depth control. You should have two 3pt levers. The outboard lever is to raise and lower the tiller, the inboard lever is to set the tiller depth for automatic depth control.
If you have the cable hooked up at the tractor end, try this out. With the tractor running, tranny in neurtal, the pto off, and the brakes locked, put the inner lever at about the mid point. Then go back behind the tiller and lift the dirt flap. The tiller should lift up. If it doesn't, then move the handle to the other notch position and try again. You should be able to see the cable move the linkage up by the control levers. You may have to play with the switches etc. on the fender also.
Is that as clear as mud or what!!??
Greg