The XFW10 is a very small wire ended pentode and very probably was designed for hearing aid use. The 22.5 Volt anode specification fits with the small dry cells available.
The filament voltage of 0.675 is half the dry cell voltage of 1.4 Volts so enabling a pair of valves to operate in series across the filament battery.
The wire pattern is known as a B5A base.
The thin glass tube envelope is oval in section 9 mm by 5 mm. The length excluding wires is 30 mm.
Reference: 1040. Type XFW10 was first introduced in 1949.
Absolute Maximum Operating Conditions¶
Updated March 10, 2013. shape:thin.glass.tube construction:all.glass type:pentode age:1940.1950 base:b5a pins:5 pin:1.a pin:2.g2 pin:3.f+ pin:4.g1 pin:5.f-.g3