The 420LFR is not listed by Thrower and so our reference is the paper label on the Bakelite base cap.
The third style of BBC stamp appears in red on the glass.
The operating characteristics on the paper label.
The label also acts as a protection for the maker.
The company was founded by D V L Fellows in 1916.
The top of the valve has blue paint around the pip. In the advert above the blue paint was used to denote a valve suitable for RF stages. The label on this valve gives the use as for RC stages - usually only for the audio parts of the set.
All Louden valves featured helical wire anodes.
Here the helical grid can be seen within the anode wires. What is of greater interest is the fact of two filament wires look to be entering the grid winding. The gettering makes examination of the valve difficult but under strong illumination and using a magnifying glass the filament in the centre of the anode is in fact a single V and this image shows the mid point. At the other end the two filament connections are close together making the filament close to two parallel wires within the grid.
The balloon envelope is 48 mm in diameter, and excluding the B4 base pins is 91 mm tall.