This HL210 has straight cuts in the base pins. The identification is present as feint etching on the dome of the envelope.
The HL210 is a general purpose battery triode with a two volt filament.
The gettering obscures the details when seen face on but looking up from the base the internal structure becomes visible. The top support is a rectangular mica sheet that does not touch the envelope. The presence of the top mica and lack of blackening of the glass, indicates that the filament was not activated by the Azide process.
In this image the grid and filament are clearly seen.
The balloon envelope is 45 mm in diameter and, excluding the B4 base pins, is 90 mm tall.
References: Data-sheet. Type HL210 was first introduced in 1929.