The provisions of this subchapter requiring inspection of the slaughter of animals and the preparation of the carcasses, parts thereof, meat and meat food products at establishments conducting such operations for commerce shall not apply to the slaughtering by any person of animals of his own raising, and the preparation by him and transportation in commerce of the carcasses, parts thereof, meat and meat food products of such animals exclusively for use by him and members of his household and his nonpaying guests and employees; nor to the custom slaughter by any person, firm, or corporation of cattle, sheep, swine or goats delivered by the owner thereof for such slaughter, and the preparation by such slaughterer and transportation in commerce of the carcasses, parts thereof, meat and meat food products of such animals, exclusively for use, in the household of such owner, by him and members of his household and his nonpaying guests and employees; nor to the custom preparation by any person, firm, or corporation of carcasses, parts thereof, meat or meat food products, derived from the slaughter by any person of cattle, sheep, swine, or goats of his own raising, or from game animals, delivered by the owner thereof for such custom preparation, and transportation in commerce of such custom prepared articles, exclusively for use in the household of such owner, by him and members of his household and his nonpaying guests and employees: Provided, That in cases where such person, firm, or corporation engages in such custom operations at an establishment at which inspection under this subchapter is maintained, the Secretary may exempt from such inspection at such establishment any animals slaughtered or any meat or meat food products otherwise prepared on such custom basis: Provided further, That custom operations at any establishment shall be exempt from inspection requirements as provided by this section only if the establishment complies with regulations which the Secretary is hereby authorized to promulgate to assure that any carcasses, parts thereof, meat or meat food products wherever handled on a custom basis, or any containers or packages containing such articles, are separated at all times from carcasses, parts thereof, meat or meat food products prepared for sale, and that all such articles prepared on a custom basis, or any containers or packages containing such articles, are plainly marked “Not for Sale” immediately after being prepared and kept so identified until delivered to the owner and that the establishment conducting the custom operation is maintained and operated in a sanitary manner.