Enneadecahedron

Guy [24] and Knowlton have independently discovered a uniform unistable enneadecahedron.

Klamkin noted that [1] contains an improved solution by R.K. Guy to the polyhedron problem, consisting of a doubly truncated 17-sided convex prism, making it an enneadecahedron (19 faces). […] Ogilvy referred to his book [4], where he describes Guy's enneadecahedron.

Our solid is a 17-sided prism, half of whose section is illustrated in Fig. 5, truncated obliquely as in the side view of Fig. 6, so it is an enneadecahedron.