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 construction matches the Mullard valve with small straps connection the anode plates.
The grids in close-up. The space between cathode and anode is filled with the grids.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 44 mm tall.
References: Datasheet & 1040 Type 6F12 was first introduced in 1948.