cover 1876 (10 May) neat envelope to U.S.A., bearing 1863-64 CC 4c. with wing-margin at left and 12c., cancelled by two strikes of the "S2" in blue, showing "HONG KONG/PAID ALL" c.d.s. in red, San Francisco transit c.d.s. in magenta and, on reverse, blue despatch c.d.s. Rare and most attractive. Photo