Symphony Valves are a new British maker to us and Thrower makes no mention of them or of a JT2.
The simple split base pins hint at early 1920s but the low filament consumption points to mid 1920s. We would love to know more about this valve.
The triode has an open rectangular anode and is mounted vertically with stout supports fixed into the glass pinch. The anode is held on one side and the grid is held on the other side.
The base has the letters KTD at the centre.
The filament is a single inverted V and can be seen to be oxide coated. The filament tension spring is attached to a support that comes up the front of the anode. The support seen here on the lower left of the image goes to a tab on the top of the valve but the silver gettering obscures the details.
This is the best image we could obtain of the two top supports.
The balloon envelope is 42 mm in diameter and, excluding the B4 base pins, is 80 mm tall.