Since this user doesn't want us to post comment at his 30 Watt Germicidal Lamp at 30_W_GERMICID_,,F___IMP_II.jpg, I'll use this as my comments to his specimen.  As you can see the coiled emitter that you see through between the envelope, I suspected that as the bulb ages as more emission materials sputtered out of the filaments and the material is trapped in the magnetic field of the coiled area of the filament travels around the field until it hits the wall of the evelope.  That is how the unique blacking pattern shows up.

Comments/Rebuttal?

Magnetic field cause forces only on moving charges, so i don't think his would be the reason. Moreover the current trough the filament is very small to cause any significant forces.
I think the only reason for coiled coil filament is to have it physically small and compact, so the arc has nowhere to move along, so the discharge would be way more stable. And it keep space for the cathode guards to be placed around the cathode (here in the form of prolinged pieces of the filament holding wires)