As memo serves that's a kind of the lock button to prevent the top and bottom parts of the lamp of getting unfasten themself accidentally. Also just to help the user with dismounting the lamp in order to change the bulb. It was used only for these of the type metal body you have. Those of the bakelite/ebonite casing had the parts taken apart by twisting/unscrewing.
Ok thanks for that, and i guess you can get the spare red lenses from Spitfire Spares or someone? and i guess the top would slide off to insert the bulb (it's a lovely thing and i don't want to damage it). Just started collecting afew cockpit bits and bob's, we went to a shop on Steep Hill, Lincolnshire and my wife n i were both in awe of some of the gauges etc and they are so tactile....... slippery slope ahead me thinks!, back to Lincoln in 2 weeks ha ha ha.