The 6F12 is the Mazda version of the ubiquitous EF91 of the late 1940s. The 6F12 is a high gain, high impedance, high slope, screened pentode.
The 6F12 was used in the 1948 Murphy V114 television receiver for RF and IF amplification.
This example seems to lack the internal coating that reduces the out gassing of the glass due to electron bombardment. This clear glass shows the anode shape.
The inner grids pass flat across the cathode.
The bright anode is approximately the width of the screen grid.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
References: Data-sheet & 1040 Type 6F12 was first introduced in 1948. See also 1948 adverts.