Dead Letter Office, Package Tracking Seal, Type 4, blue. With red manuscript DLO record number, affixed on 2¢ green (W316) wrapper cancelled by "Baltimore Md 22" double oval to Frankfurt, Germany, "Baltimore, Md. F.D. May 1 1900" cds and handstamp "48" in circle struck, purple "Unmailable, Subject to Letter Rate of Postage" with blue crayon "12" and matching purple handstamp on reverse "Sent to D.L.O. Baltimore, Md. May 5 1900", Very Fine and most unusual, one of three recorded type 4 seals used on cover. Scott No. OX  Estimate $500 - 750. $0 (Image)