Osram - GEC Nitrogen spectral/calibration lamp

Something very unusual I picked up on Ebay recently. Whilst resembling a lamp with its B22 base I suspect this is in fact a type of radio valve or similar. I'm not sure whether "N2" is simply a type/code or whether this is filled with nitrogen? The base states that the lamp is not to be used without a series resistor. (Edit: the purpose of this lamp has been uncovered thanks to some valuable information supplied by James! He writes: "They were made by GEC for Adam Hilger, manufacturer of some of the finest optical apparatus and spectroscopes in the world. The discrete spectral lines produced by various gas discharges were used for calibrating the equipment.")