Obviously there has been (at least in 1955) an international agreement for forwarding letters from one country to an other and have them sent free of charge via a state owned colleague institute. In this case a letter of the United States department of Agriculture/Official Business had to be sent to a commercial bank in Utrecht.
First it was stamped in blue: Forwarded through the/International Exchange Service/Smithonian Institutions/Washington D.C./USA/Record 6845. On arrival in the Netherlands it was stamped: De bibliothecaris/der/Koninklijke Bibliotheek/No. 615 and DIENST/Ministerie van Onderwijs,/Kunsten en Wetenchappen. Then it went through the normal mail and got the machine cancel ‘sGravenhage 7.3.1955. Any of the readers knows more of this Internationale Exchange Service?