Thanks for finding!
Must’ve been something only they knew. Maybe the card was sent with just one of the item in question.
Early ‘geocaching’?
Maybe “The Challenge” was to find a matching pair of postcards. 😄
11 Cradock St, 11 -— name is of Welsh origin meaning beloved, amiable, love
Maybe someone was having a little fun by dropping it in the mail at this time, 11 x 11 years on.