The 6F23 is a screened amplifier pentode. The presence of two cathode connections suggests an RF role where lead inductance needs to kept to a minimum.
Thanks to Dave Harding for the following information. The 6F23 was used in the late 1950s and early 1960s as an IF amplifier in Ultra TV sets. Ultra always used Mazda valves (30P4/30P19 line output 30PL13 frame) at the time. It was always felt this was because they were produced 'in house', Ultra and Mazda being part of the same company. The valves were near equivalents of the popular Mullard EF80/EF85 - PL36 and PCL85 though not pin compatible. As an afterthought Mazda valves were very unreliable compared to Mullard and Brimar was somewhere in between. (Not that the PCL85 was an icon of reliability.)
The screen makes observing the inside difficult but control grid copper supports and radiating fins can be seen.
Made in England and BVA. Within the screen can be seen the straps that hold the two anode plates either side of the central grids. An open pitch wire can be seen indicating a true pentode with a suppressor grid not a beam plate.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins, is 58 mm tall.
References: Data-sheet, 1040 & Dave Harding. Type 6F23 was first introduced in 1959.