Great replies folks. I was thinking about making mine almost like the John Deere style, just gonna add a spring so that the square on the bolt head will always stay engaged in the slot. I did know about the knob and lever under the seat but I will need to be raising and lowering the 3PH a lot when tilling or drilling. My wife and I will be planting a bunch of laurel bushes and each one needs a hole about 1 foot deep. When I plant taller cedar trees I drill about 2 feet deep just so there is lots of loose soil for the roots to go into and so that the area will drain well. I'm glad that Yanmar actually made a stop because this means that the 3PH positioning should be repeatable. Now I need to look for some sort of weatherproof stick on scale, like a section of a tape measure, to stick next to the slot so I can equate numbers to drilling and tilling depths.
Thanks,
Eric