Here are a couple of photos of the standard armboard for the KD 500:
I used that armboard with my Grace 940 arm. I am now using the board cut for the SME. Both came with the turntable.
Sorry I can't part with the standard armboard as someday I could take a notion to use a different arm on the machine.
I measured this part a while back and posted the numbers. The part is stored in the attic now. If necessary and if I can't find the old post I can go up and get it and repeat the measurments.