| http://www.w3.org/ns/prov#value | - must be strong and the contents double packed in materials such as cardboard or plastic.The seals on an ordinary envelope should be placed as shown below:Insured letters which have the exterior appearance of a box must fulfil the following additional conditions:(a) they must be of wood, metal or plastic and sufficiently strong(b) the walls of wooden boxes must have a minimum thickness of 8mm(c) t
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