(20d) 1866 3k black & green, printing error of background, "V" (for 5k) instead of "3", block of four, unused with full original gum, h.r., with minor perf. strengthening, remarkably fresh and well centered for issue, v.f. One of only three known blocks of four (Mi.19xF) (YT 19A) (Zverev $11,000)