[Mondial]

If the shadow is not symmetrical, the filament of the bulb may not be aligned with the light aperture on the back. Philco originally used a special bulb with a precisely located filament. If you substitute a normal 44 bulb, the filament may not be located properly with respect to the aperture. The filament should be directly behind the hole on the back of the coil assembly. If it is not, you will experience an asymmetric deflection of the shadow. 

You may have to try several bulbs of different brands to find one that works well.