We have no idea what this is or what it is for but the two parts seem to be related.
"The one on the left looks like it could be a master plate for what is called a super spacer. It is used in the machinist trade for accurately spacing holes or slots in a rotary indexer"--input from another ebayer.
Outer diameters are about 7 inches. There are some markings but they don't mean anything to us.
Additional similar or related items may be available listed separately or on request