The spines were, by the early 1960s, the only part of a Trench pennant that was still 100% wool. Whatever that woven cloth was made of that was being used for their pennant bodies, didn't appeal to critters at all. But it sure faded easy; and it definitely frayed.
Even the 70/30 wool/rayon blends, which Collegiate of Ames and Chipenco had moved to by the 1950s for their pennant bodies, held up surprisingly well to critters. The Rayon must've been poison!