The XFY21 is an audio output valve for use in hearing aids. The HT would be 22.5 Volts and the cathode current 0.48 mA. The output delivered to an earphone would be 1.8 mW.
The B5A/F base has pin 1 by the red spot.
The Hivac name on the glass. Within the beam plates are bright and the two wire grids can be seen. The filament is the fine wire in the centre.
A closer view.
The thin glass tube envelope is 6 mm x 9 mm and, excluding the B5A base wires, is 37 mm tall.
