Bakelite case.
In "Collectable Clocks" (Reference: p 392-4 of CC), Shenton has extensive details of the "Reform" movement manufactured by Schild and Co. and states "these movements proved to be extremely reliable and found their way into a number of casing variations...they were later developed for use in the delayed action mechanism of marine mines".See c72 for such a movement
Pictures kindly provided by Jon Gilbert