This exhibit is unmarked on the glass but the underside of the base does carry the Metrovick logo. The valve matches the Cosmos SP-16 design and that is probably what this valve is. The SP-16 came in three sub types but it is also possible that this was an early example.
The construction, size and flying lead are all characteristic of the SP-16.
The grid is attached to a stout support rod on one side only.
The folded bright anode is spot welded to the support.
The balloon envelope is 43 mm in diameter, and excluding the B4 base pins is 98 mm tall.
Reference: 1043. Type SP-16 was first introduced in 1928.