cover 1920 registered envelope to Philadelphia (3.6 arrival) bearing 6c. on 3c. block of four cancelled by oval "U.S. POSTAL AGENCY/R.C./SHANGHAI, CHINA" h.s. and showing on reverse "U.S. Postal Agency Shanghai, China/REGISTERED" double-ringed d.s. (8.5) in violet. Fine, block of four paying the correct registry rate is scarce. Sc. K3. Photo