  New historical aspects of the classification, by Cayley and Cremona, of ruled
quartic surfaces and the relation to string models and plaster models are
presented. In a `modern' treatment of the classification of ruled quartic
surfaces the classical one is corrected and completed. A conceptual proof is
presented of a result of Rohn concerning curves in $\mathbb{P}^1\times
\mathbb{P}^1$ of bi-degree $(2,2)$. The string models of Series XIII (of some
ruled quartic surfaces) are based on Rohn's result.
