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.

 
Pin Connections

B5A	1	2	3	4	5
	a	g2	f+	g1	f-,g3

 
Absolute Maximum Operating Conditions^¶

Pentode	Vh	Ah	Va	Vs	Vg	mAa	mAs	ra	gm
	0.0675	0.025	22.5	22.5	-0	0.2	0.01	1M	0.18