The HX210 from Double 2 is a directly heated triode and looks to date to the 1920's or early 1930's. The information we have comes from the etched identification on the dome.
The pinch and little else except the gettering.
Looking up from the base reveals the horizontal anode and grid winding.
The balloon envelope is 45 mm in diameter and, excluding the B4 base pins, is 85 mm tall.
