The L77 triode was specified as both an amplifier and an RF power valve. In the latter role it would deliver 5.5 Watts. With a very small envelope volume, this would make it ideal for hand portable equipment. In larger apparatus it could deliver enough drive power for much larger PA's.
The anode is a blackened flat sided box with ribs pressed into the sheet metal to give extra mechanical stiffness.
The face view. the side flanges aid radiation and provide a flat area for the fixing notches.
The thin glass tube envelope is 18 mm in diameter and has an overall length, excluding the B7G base pins, of 48 mm.
References: Datasheet & 1040. Type L77 was first introduced in 1951.