"Nickle-colored plate" fastened to the rear of the main body(inside the float bowl) is the rear metering plate, and those are "clutch head" screws that are holding it to the main body. You will need to remove this plate in order to clean the old metering plate gasket off the main body, this gasket also serves as the float bowl to main body gasket at its outside perimeter. There will also be a thin steel plate and thin gasket the same shape as the "nickle-colored" plate, stacked together on the larger metering plate/main body gasket. The metering plate, thin gasket, and thin steel plate can be left together if they look intact, these rarely go bad, but you have to remove this assembly to remove the main metering gasket/combo bowl gasket.
If you don't have a clutch head screwdriver, find an old small phillips head screwdriver that will fit fairly snug after you have "flattened" the point with a file or bench grinder. It will get you by in a pinch.