It means that every strap was made of two strips of leather sewn together. In the chain of narrators of this hadith there is a narrator Khaalid Hadh-dhaa. Hadh-dhaa in 'Arabic means a shoe maker. The 'ulama write that this narrator was not a shoe maker, but had an acquaintance with those whose trade was shoe making. Due to his acquaintance and friendship, he was also named (Khaalid) Hadh-dhaa, and became known by his name. one is surely influenced and develops the habits and manners of those with whom one keeps company.