The EL12N is an audio output valve. It is described as a pentode but is constructed as a beam tetrode. The EL12N is closely related to the older EL12 and the EL12Spec but not pin compatible.
The makers logo for RSD indicates a German reseller, based in Frankfurt, that specialises in valves from the former Eastern Bloc countries.
A view of a new valve but with little detail visible.
The construction uses a pressed glass foot and the electrodes are welded to the tops of the inside portion of the compound base pins. The outer portion is probably a wire that is soldered to the external base pins. The beam plate can be seen within the anode.
Thanks to Mark Mallison for sending the image.
The wide glass tube envelope is 42 mm in diameter, and excluding the footless base pins is 93 mm tall.
References: Data-sheet. Type EL12N was first introduced in 1956.