DeltaRed: I am just west of Dubuque, Iowa.
As for the different straw walkers. I looked in the PC580 and it calls the ones that are flat "Maize" walkers and the stepped ones: Grain, Rice, and Rice crawler.
Then in the newer parts catalog PC656 they just are called: Flat walkers and where only used up to serial number 35,000.
They are not listed in the index you just go to straw walkers and then go to the next page.
Until you posted you had some I had never seen or heard of them. It looks like they where for corn. I would look in an older Operators manual and see if they talk about them. I will try to see if I have the old round back manual. I wonder why a Rice machine would have them in it??? Because it looks like they where just for corn.
I ran a JD 95 for several years. It was a square back machine. I am not sure what manuals I have for them.