Just found this on our old combine shed (built for the Massey 90 back in the mid 1950's), it's just over 1/8 inch thick, made of steel. There are traces of black paint around the rust and carbon along the inside edges for each cylinder. Is is a spacer to reduce compression with different types of fuels?
Come across one on a 6cyl John Deere header some 40 years ago, petrol/kero engine and it was about 1/2" thick.
I had one of these for a Model T. It was part of a kerosene burning setup which included special manifold and carb