1856 (Jul. 25) Madison Wis. to Arendal, Norway, buff cover bearing 12¢ black (17) block of four, deep shade with sharp impression, tied by three strikes of blue "Madison Wis. 25 Jul" cds, red "New York Br. Pkt. Jul 29" exchange backstamp and matching red "38" cent credit handstamp overstruck by red circular grid cancel and re-credited "23" cents in magenta manuscript, carried by Cunard Line Niagara from Boston Jul. 30th to Liverpool Aug. 11th, red Aachen Franco (8.12) framed transit and blue manuscript "6" sbr rating, Danish (8.13.56) and Hamburg (8.15) backstamps; block has framelines and margins skillfully added at bottom and upper left to improve its appearance, Very Fine appearing and extremely rare use, ex-Sevenoaks. Estimate  $5,000 - 7,500. ONE OF ONLY SIX 12¢ 1851 BLOCKS USED ON FULL COVER, THIS BEING THE ONLY ONE USED TO NORWAY. According to our records the six recorded 12¢ 1851 blocks used on full cover are:1. 1856 May 13, New York N.Y. to Spezzia, Sardinia.2. 1856 Jul. 25, Madison Wis. to Arendal, Norway, ex-Sevenoaks, cover offered here.3. 1857 Feb. 4, Wheeling Va. to Wurttemberg, Germany, block of five, ex-Zoellner.4. 1857 Mar. 10, Bristol R.I. to Birmingham, England5. 1857 Aug. 3, Nashua N.H. to Valparaiso, Chile, ex-Gibson, Grunin.6. Unknown, New York N.Y. to San Francisco, block of 18 on a large legal size cover, ex-Tracy, Jessup. $0 (Image)