Leather could be the wolf pelts, but the hair has worn away or fallen out over time, or like others suggest, while on the Great Hunt they came across a mighty beast, the Beast slaughtered many, but was eventually taken down by the vengeful Fenrisian warriors of Sesc... as a tribute to the fallen, the surviving warriors of Sesc each took a piece of the great animals hide and inscribed runes of saga on to it. The more heroic the warrior the more hide he received with which to commemorate his glory. The remaining hide was put in stasis to be given out each time a warrior carried out a deed worthy of legend.
As new warriors joined the ranks of Sesc and proved themselves worthy, they are each awarded a small piece of the great beasts hide, on which to inscribe their own runes of saga. Over time if they prove worthy they will have many pieces of leather, stitched together to create their own legends.
Wolf pelts became a sign of prestige and status amongst the Great Company, believed to bestow the spirit of the wolf to its owner. These pelts are only reserved for Jarls and Thegns who have truly ascended to greatness....
PS your models and paint work are freaking awesome