Found this reference regarding suspended swords on the back:
However in "The Ancient Celts" by Barry Cunliffe, on page 94 of that book, Professor Cunliffe writes,"All these pieces of equipment [shields, spears, swords, mail armour], mentioned in the texts, are reflected in the archaeological record and in the surviving iconography, though it is sometimes possible to detect regional variations. Among the Parisii of Yorkshire, for example, the sword was sometimes worn across the back and therefore had to be drawn over the shoulder from behind the head."