OSRAM 12V 3W

I bought a vintage Tandberg TR-2060 AM/FM receiver a couple of months ago. I've placed Synergistic Research ECT's inside at multiple appropriate places and the sound quality takes me back to my teen years when spinning vinyl was commonplace. There's a rich meatiness that's missing from my SS Primare pre and power amps. I plan to replace 2 fuses with Synergistic Research purple fuses. There was also an OSRAM 12V 3W inside (what I thought was a 3rd fuse). It lights up when the unit is turned on. But in researching the OSRAM I've found it to be called a light bulb. Is anyone familiar with this "bulb?" What is its purpose? I guess as long as it's lighting up I should just leave it alone?