This is a used GM differential auto adapter plate cover insert – part number 12554838 – for the 1997 through 2004 Corvette C5.
If you're rebuilding your diff or swapping out the cover, this is the factory piece that goes between the transmission and the differential.

It's a used part from GM, so it may have some wear or old gasket material stuck to it – the seller notes that you'll need new seals when you install it.
That's pretty standard for any diff work anyway.

Perfect for a budget rebuild or if you lost your original during a project.
It's a simple metal plate, but it's critical for sealing and alignment.