Chris — The similar sized boat, Silber & Fleming No. 5029, has a gadrooned base matching the sides, and no maker's marks. It is shown in the Silber & Fleming wholesale catalogue on a crystal rod stand, held together with silver-plated brass wire, resting on a long mirror plateau on a maroon velvet covered wooden base. Silber & Fleming sourced their goods from a variety of manufacturers in Britain and Europe. We don't know whether they bought in 5029 complete or bought in the components, assembling it themselves in their own workshop. Certainly the mirror plateaux were manufactured by the same British manufacturer as supplied most or all of the mirror bases for epergnes &c., as the standard pale green and creamy yellow paint on the underside shows.
Your opalescent version with the hobnail base is new to me. One possibility is that Silber & Fleming had to go to a different manufacturer to obtain an opalescent version, necessitating a new mould. The catalogue illustration shows only a side view, so any base pattern would have sufficed, and a different pattern would have reduced the risk of being accused of plagiarism.
Bernard C.