I put it in my photo thingy and blew it up. All I can tell is that what ever it is, it is no good anymore.
As to exaust gaskets, I have done a great deal of engine repair on both diesel and gas from chevy and briggs to cummins and detroit but I have never seen one exactly like it.
The absence of bolt holes indicates a possible exaust dougnut, but I cannot call it for sure.
yeah, the first thing I did was to blow it up. it does indeed appear to be a 'leaky' gasket which .
Notice the irregular 'tab' on the bottom left, and the very regular right side, it could have the been detached part of last cylinder port of standard exhaust header of a gasket for multi-cylinder engine with a standard flat header rather than a donut type individual headers.