The 5703WA from Raytheon, is a post WW2 experimental type.

Some other subminiature valves in the '57' range are clearly for military purposes (Type 5768 is listed as a 'rocket triode'! Probably designed for use in proximity fuses.).

One advantage of the very small size and weight is robustness to vibration and shock. Such a small valve would get very hot at the ratings indicated but it might not be required to operate for more than a few seconds.

The thin glass tube envelope is 9 mm in diameter and, excluding the base pins, is 38 mm tall.

 
Absolute Maximum Operating Conditions

Triode	Vh	Ah	Va	Vg	mAa
	6.3	0.2	120	-2	9.0

Thanks to Frank Philipse for supplying the above datasheet

This exhibit was last updated on 02 July 2007 {find valves by name}