The 6F15 is an early all glass variable μ pentode. It would have been designed for AGC controlled RF and IF amplifiers.
The mutual conductance is a function of the winding pitch of the control grid. In variable μ types the grid would be wound with a graded pitch from fine to course.
The B8B base in its original form with metal skirt with locating pip and spigot. The locating spigot is 13 mm long.
The top of the electrodes can be seen through the getter holder. The cathode is a round tube and the heater is formed from several insulated loops.
The bright anode can be glimpsed within the perforated screen.
The thin glass tube envelope is 20 mm in diameter and, excluding the B8A base pins, is 53 mm tall.
References: Data-sheet & 1040. Type 6F15 was first introduced in 1948. See also1948 adverts.