In have one. Don't use it much. Greg at Reeder makes it for his machine. Check his site.
Oh…I regularly check Greg’s site (Reeder products) to drool after the few bits and pieces that I haven’t already bought!

However, he is a bit too far away for me to buy more of his stuff on whims. Shipping, tax and import duty add up!
What do you use it for when you do use it?
I have been wanting to have a tool rest that has a recess for the vertical platen, in order to grind with a push stick , spine supported on the rest. Thought if I was going to make one for that, might as well be long enough to turn around and use horizontally with the wheels.
Don't have one, but you could use the holes for lots of things...
Changing the orientation of the table, setting up jigs, installing a lever to use as a push stick to increase grinding pressure. Installing angled rests for pre-set grind angles.
Probably lots more... Sky's the limit! Just gotta have some imagination.
Uncharacteristically, I am running an imagination deficit!

Had thought of the pressure lever stick, but not the angled guide fence, like that idea. Not sure what kind of jigs would benefit from a fixed base, knife making or otherwise. Though it might be interesting to hear from others.
Grinding arcs would work with a fixed pivot point, I did something like that on my old grinder.
Cheers!