The L2 is a low power audio amplifier triode. The filament is probably oxide coated to achieve such a low heating power.
The etched number is 1015. The date is 17 May 1934.
The grid has the top supports held in a glass bead that can be seen in front of the ribbed bright anode.
Side view showing the depth of the anode and the glass supporting bead.
The dome carries the identification. The Mazda name is clear the L2 at the top less so. The etched lettering around the circumference of the dome reads: GPO Research Station NW2. This is the centre at Dollis Hill.
The balloon envelope is 45 mm in diameter and, excluding the B4 base pins, is 90 mm tall.
References: Data-sheet & 1040. Type L2 was first introduced in 1931. See also1931 adverts.