The Marconi HL210 was first introduced in 1928 and is a general purpose triode.
The etching on the face of the envelope is hard to read. It says Marconi Valve and has the BVA logo beneath. The base has an A to indicate the anode pin.
The pinch and the wing on the base also indicating the anode pin.
The etching on the dome is most distinct.
Looking under the gettering the horizontal box anode is revealed. The grid is just visible at the top on the picture.
With enhancement the grid is easier to see. The anode can be seen to have been closed with a folded seam.
The balloon envelope is 40 mm in diameter and, excluding the B4 base pins, is 73 mm tall.
References: Data-sheet & 1043. Type HL210 was first introduced in 1929. See also1929 adverts.
