The Fender Musical Instruments Corporation of America was started by Clarence Leonidas 'Leo' Fender in 1938 and is famous for electric guitars and amplifiers. This 5881 / 6L6 would be made by a third party and branded for them.This valve has glass disc construction that is fitted to an octal base. It is a reasonably powerful audio beam tetrode and was used for audio amplifiers.
The box anode has the side flanges at the centre of the working faces. The end seen has openings through which the inner grids can be seen.
Looking into the base, the glass disc construction can easily be seen.
The top mica. In the centre is the heat radiating fin for the control grid.
The wide glass tube envelope is 33 mm in diameter and, excluding the IO base pins, is 72 mm tall.
References: Datasheet & 3002. Type 5881 was first introduced in 1954.