In apparatus for and a method of inserting a series of contents (1) into a series of containers (3), identifying magnetic indicia are printed on each one of the series of contents before insertion in the containers. After insertion and sealing of the containers the contents cannot be visually inspected without breaking the seals. The indicia are magnetically read by a reader (7) through the material of the sealed containers (3) and visible indicia corresponding to the identifying magnetic indicator retrieved from store (6), and applied to the exterior of the containers by a printer (5). The apparatus and method are applicable to automatic mailing systems where it is necessary to ensure that the correct addresses corresponding to a previously inserted series of letters in a series of respective envelopes are printed later on the exteriors of the envelopes. Instead of magnetically based systems, infra-red radio isotope or radio frequency identification based systems may also be used.