The two knobs I got were star knobs rather then the round knobs that the machine came with.
Those star knobs don't work well for me due to arthritic conditions in my thumb joint. I sort of solved this by exchanging the knobs with an extension table that I don't use much thus giving the main table all the round knobs.
I've tried to find a replacement for a while but it was never a very high priority. Recently this subject has come up on the forum so When I placed an order from Lee Valley I ordered what looked to be close to the right knobs. The pair arrived so I was able to check them out and yes the round plastic bit looks to be the same thing. However the knob does not have the same lower profile and the screw length is to long, or is it?
In this next image the knob on the left is the original one from my 520. The middle one is the Lee Valley knob with a 1/4" nylon bushing, making it pretty much the same as the shopsmith version. On the right is the knob as it arrives.
If this interests you the Lee Valley part is a 1/4"-20 1" Clamping Knob (Male 1" stud) and they cost $2.30 in low quantities. The spacer/bushing I got a while back at a hardware store so I don't have a cost for them.(5 or more $1.95 each)
https://www.leevalley.com/en-us/shop/ha ... em=00M5621
I seem to recall the shopsmith part is like $4.95????