horizontal pair (positions 24-25), left stamp with inverted "M" in "Panama" (13.6), right stamp with "Panama" 16mm long (13.8), tied to registered cover to Dr. Perry in Ancon, April 24, 1906 Cristobal datestamp, backstamped Ancon the following day, very fine and choice pair on cover, just 400 of each variety produced, ex-Plass