The N78 is a 1953 audio output pentode with a high mutual conductance, suggesting that full output can be obtained with a low input signal.
In class AB1 push pull a pair of valves can deliver 9 Watts at 4.5% distortion. If connected as triodes the pair can deliver 6.3 Watts at 1.6% distortion.
M-OV produced these valves and branded them with paper labels: GEC for professional sales and commercially, Emitron, Marconi and Osram.
The Osram label.
The suppressor grid support can be seen through the hole in the end of the anode. The support is notched by a pointed disc cutter, the wire is dropped into the groove and finally a flat ended disc pushes the displaced metal back and this locks the wire in place.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 59 mm tall.
References: Data-sheet & 1040. Type N78 was first introduced in 1952.