1956 "OPEN PANEL" CACHET. Window envelope from London with 4d meter mark dated 17 VII 56, window unsealed and thus cachet "RETURN TO SENDER/OPEN PANEL (F.S) CONTRARY TO REGULATIONS SEE PAGE 111 P.O. GUIDE". On reverse "OPEN PANEL RETURN TO SENDER INADMISSIBLE FOR TRANSMISSION ABROAD" cachet. Very unusual. (Ref 87196).
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