What is this tool used for?

[kkkcj]

What tool is this? This was in a Sioux valve grind set But I can't see how it was used. Guide or seat concentricity measurement?

 

[engineer2]

I had a mystery Sioux valve grinder attachment and I had good luck searching for the model number online and then selling it on GJ.

 

[Ricky Joe]

Ford flathead valve seat grinder, for getting the seats centered and concentric. There is a dial gauge that goes with it to check concentricity.