Seems like a truncated MG part number 976139, which is the correct OEM gasket for the Calvin. I stopped using these on my bike for two reasons: leaks within 4K miles each side, and the fact that they stick to both the valve cover and the head like glue regardless of whether you grease them prior to the installation or not.
I use part number 14023760, the same thick green gasket that's used on square fins prior to the Vintage.