China 1941 Chungking Custom House On Service Envelope Airmail Sent to the United States via Hong Kong Bearing $1 “Thrift Commemorative” Scott 470 Single Franking for Airmail; with Original Letter from an American Employee of the Chinese Customs Discussing his Work & his Imminent Trip Down the Burma Road as a Member of the Investigatory Commission; An Exception Item with RARE Single Franking of the Thrift Commemorative; RARE & FINE (46-3499)