Nice find!
First, Congrats on finding that classic machine. Not too many of them around anymore.It took the nurse a while to sell it but I figured someone on here would grab it before it went to the crusher!
About the strainer pics... the course strainer is upside down. It goes the other way! I would hate to have you put it back wrong! LOL
About the wash arm support and design changes...
They changed the design of the 16 and 17 support because they changed the sump and heater configurations a bit. They used the original 16 support in the WM-1 series and then switched to a shortened version of it and did away with the 2 piece filters as used on the 16/17's most likely because the harsher and more alkaline commercial detergents would eat through the plastic.And warranty costs when they first came out and users breaking them as well would have made them rethink the design. They went to a one piece stainless steel filter assembly and lowered the support height to match up with the stainless filter. But they also removed the sealing ring that was below the opening of water discharge. You can all check this on your K/A's as they all have them in some way shape or form up thru the 23's.
This allowed water to "leak" down a bit an lessened the pressure out of the washarm. I know for a fact that the WM's would easily toss cups and glasses around so they needed a fix and that probably did it! I am sure anyone with a 15,16, or 17 will attest to the power of that Hydrosweep and how they will find bowls and such overturned in the bottom rack.
As far as the rust stains are concerned.. SImply put, the water that used to flow into that machine was loaded with iron and stained the machine. Like Ed mentioned..some soft scrub with bleach should do the trick nicely without damage or try using some ZUD which has oxalic acid and will definitely remove the rust. If you have good water in your area, it should not come back.
Coincidentally, the owner's manual we had with our KDS15 actually mentioned the Oxalic acid treatment and I remember even using it on my aunt's pull out GE d/w that was orange inside from the water in Oradell, NJ. Nothing worked to make it look nice till we threw 2 tablespoons of the Oxalic Acid into it and ran a full detergent free cycle, and Voila! brand new inside after that!
I think that gets all the mysteries solved so far!
Good Luck with it. Should last many more years. Just get yourself a new washarm support as they were discontinued many years ago.
Hope this helps.
Steve