I don't know that it was ever actually issued as a postcard, but the drawing you seek does appear on page 17 of Richard's book, ZEICHNER UND "GEZEICHNETE" (Berlin, 1912). Richards was a German cartoonist who met Twain in 1907, and the first 11pp. (pp. 9-20) of his book concerns Twain, with various drawings, photos, and facsimiles. There are usually several copies listed for sale on the net, usually at modest prices, but not in collectors' condition, but perfectly fine for research. Kevin Mac Donnell Austin TX