Replace lamp in old amplifier
Just rotate the plastic from the back (CCW to "unscrew", it's sort of a bayonet base-like action). I have the same general kind of lamp in the instrument cluster of an E28 chassis automobile. The metal contacts work against pads on the single-sided PCB. It's a 24V 1.2W bulb (photo from linked site).  





As @Jack mentions some such bulbs have removable bulbs and you can replace the bulb without removing the base from the PCB (but some are soldered in). Here is a (similar, but not exact) 12V example showing both: 



It's pretty easy to break the bulb trying to get it out, in my experience, so I buy the ones with base attached for slightly more money.