The DF64 was specifically designed for use as an audio amplifier in hearing aids. The filament is accurately constructed and is suitable for series or parallel connection, two valves would be connected in series across a 1.5 Volt dry cell. The anode supply would be a 15 Volt dry cell flat stack.
The DL64 output pentode produces 0.95 mW and runs from a 1.25 Volt filament supply. I believe that the typical line-up would be a pair of DF64s and one DL64.
The three grids can be seen through the side wall of the anode.
The Mullard name. The top mica shows the supports for the three grids.
The miniature envelope is 7 mm x 5 mm, and excluding the base wires is 24 mm tall.
References: data-sheet. Type DF64 was first introduced in 1953.