  Between his arrival in Frankfurt in $1922$ and and his proof of his famous
finiteness theorem for integral points in $1929$, Siegel had no publications.
He did, however, write a letter to Mordell in $1926$ in which he explained a
proof of the finiteness of integral points on hyperelliptic curves. Recognizing
the importance of this argument (and Siegel's views on publication), Mordell
sent the relevant extract to be published under the pseudonym "X".
  The purpose of this note is to explain how to optimize Siegel's $1926$
technique to obtain the following bound. Let $K$ be a number field, $S$ a
finite set of places of $K$, and $f\in \mathfrak{o}_{K,S}[t]$ monic of degree
$d\geq 5$ with discriminant $\Delta_f\in \mathfrak{o}_{K,S}^\times$. Then:
$$\#|\{(x,y) : x,y\in \mathfrak{o}_{K,S}, y^2 = f(x)\}|\leq
2^{\mathrm{rank}\,\mathrm{Jac}(C_f)(K)}\cdot O(1)^{d^3\cdot ([K:\mathbb{Q}] +
\#|S|)}.$$
  This improves bounds of Evertse-Silverman and Bombieri-Gubler from $1986$ and
$2006$, respectively.
  The main point underlying our improvement is that, informally speaking, we
insist on "executing the descents in the presence of only one root (and not
three) until the last possible moment".
