The V455 from Mazda is a pentode. From the first edition of the Wireless World data books onwards the V455 is not listed. This could indicate a professional type with limited application. Thrower lists a V453 as a pre-war mains pentode with a 4 V 1 A heater but he also has no details of a V455. The lettering on the metalising is clearly V455.
The anode is connected to the top cap.
In 2016 Stephen Rowley was able to contribute the following information based on his father's experience as a BTH cinema engineer.
The V455, SP41, Pen44 and UU5 were all used in the 'new' BTH SUPA 35mm film projector. The V455 was the first stage pre-amp. The valve was also the main volume control being a variable μ valve. It was supplied with -35 VDC via a 47kΩ potentiometer. There were 2 V455's, one for each projector. BTH had combined a lot of new innovations into their first new projector after WWII. The main innovations were to have everything within the 2 projectors, so one had the amplifiers and the other had all the contacts for the arc lamps. The amplifiers were built on individual trays that could be pulled out on runners like a draw. They could be turned up side down for access. There was a pre-amp tray, power amp tray, the power supply tray and a 200 VDC power supply for the stage speakers. In the small set there were 2 Pen44's in the output, the 45 Watt had 4 Pen44's and the 90 Watt used the main and stand by set together.
The SUPA valves we have details for are all 4.0 Volt indirectly heated types and thus it is reasonable to assume that the V455 also has a 4.0 Volt heater.
The BTH SUPA projector.
The classic envelope is 40 mm in diameter, and excluding the B7 base pins is 107 mm tall.