[Are all models including the center-fires of this same design with the receiver cover being part of the rifle?]
No.
The only ones designed with the receiver cover are the Ithaca/Irma/Iver Johnson clone .22 rimfires & Yellow Boys.
The Big Boy centerfires are made of a brass-like material Henry calls Brasslite.
IIRC, the brassie sideplates are removeable, since factory custom engraved/personalized replacements are optional/available.