On another thread I posted that there was a thinner block shim gasket-there was. Cat discontinued it years ago. Instead Cat makes a thinner Spacer plate. These might be old #'s, Standard plate #6I2981. The undersized #6I4303 it's .003 thinner that standard. Cat went this route and abandoned the thinner shim gasket.
This may shed more light. Spacer Plate numbers are both still good.