The A2521 is a low noise UHF triode with an anode dissipation of 2.5 Watts. The advert referenced above shows that these B9A triodes were competing with the disc seal triodes such as the DET23 and DET29.
The GEC logo is stuck on.
The electrodes are mounted horizontally with the cathode at the top.
The heater end of the cathode. Between the mica cheeks is the curved anode. The grid is of frame grid construction and mounted very close to the flat cathode tube.
Thanks to Robert Coleman for the pinout information.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 46 mm tall.
Reference: 1040 & datasheet from Rob Coleman. Type A2521 was first introduced in 1957.