6c on 5c black & blue postal stationery envelope with '1910' overprint in black (H&G B102) used with added 1916 2c red (SG 684) tied by 'Lines' cancel, however the envelope was deemed to be invalid with large '0' handstamp in blue below the stamp imprint (this envelope series was demonetised in 1913) with NEGOCIADO DEL INTERIOR SAN SALVADOR cds alongside dated 10 OCT 1918 on front. Addressed locally.