6 V / 300 mA (1.8 W), E10 socket (MES = Miniature Edison Screw, the same as E10).
These are nothing fancy nor rare, but the 6V types are usually only available in specialist electronics stores. I was once too lazy to go any farther than the nearest we-have-everything kind of store, got some 3V flashlight bulbs and modified the circuit to have the bulbs connected in series instead of parallel. These will usually have a spherical glass envelope instead of tubular as in correct marker bulbs, but it's not a problem in BM900.