I think that would be the high wheel version as I think the low wheel were about 30", just thinking, don't know for a fact. I also think it has a gear box on it with 3 speeds marked 2, 3 or 4 andas I read this chart that says approximate drilling distances No. 999 planter here goes, plates 16 cell, high wheel, first figure is setting of shift lever, second is large sprocket, next is medium sprocket and finally small sprocket.
2 13 1/2" 25" 32"
3 7 1/2" 9" 17"
4 2 1/2" 8 1/2" 12 1/2" the low wheel slightly less.
And for plates they are measured in 64" with the length first then width oand finally thickness of cell. From what I am finding and this is out of the Deere plate bookPC-268 the plate that is needed is Y 3352 B Small sweet corn 36 - 10 - 20 or possibly this Y 2637 B small corn 36 - 12 - 20 or Y 4664 B small crosby sweet corn, extra small corn 28 - 12 - 20 or Y 2636 B extra small corn 32 - 11 - 20 or finally Y 5519 B small round pointed corn 34 - 14 - 20, There are only a total of 15 different plates in 16 cell for that planter. And what was graded as a medium is now graded as a large kernal. The disk openers were an option that I have never seen that I remember as with clean working to garden finess they were not needed, the disk was more for trashy soil conditions as it would cut thru the trash instead of making the runner push thru and pile up the trash.