MAURITANIA UPU ARCHIVE RECEIVING AUTHORITY SPECIMENS: 1921 BENASERS 10ch on 6ch and 1kr on 12ch both in strips of three affixed to pieces of ledger page, without any specimen defacements, subsequently handstamped with large part 3-line "SPECIMEN / COLLECTION / MAURITANIE" in red-purple. Sent from the UPU in Berne to the Mauritania Post Office where they were affixed to the ledger page and tied by the Mauritania handstamp. Some imperfections (surface faults). Unique and philatelically important proving distribution of this issue in normal unused and undefaced state as specimens through the UPU. An excellent expertising reference for the surcharges.