I'm not sure what plastic parts above is referring to other than the butt plate, .
The fore end spacer is plastic on the 840.The 84 has a nice semi case coloured reciver while the 840 reciver appears to be Japaned black.From looking at mine I would say that the trigger guard appears to be made of a different type of metal.
The trigger gaurd and also the thumb lever to break the shotgun open are made of an alloy.



























