Item is a backing plate for mounting an Axminster chuck to your wood lathe.  This backing plate is 3-15/16" diameter and fits the older Axminster "Super Precision Chuck".  This model chuck has a removable backing plate to allow it to be used on a variety of spindle sizes.  The bore of the backing plate is 5/8" and is smooth, no threads.  My understanding is that this will fit a Shopsmith.  There is a set screw to lock it to the spindle.  The plate also has 24 indexing holes around the outer edge.