At first sight, this seems to be a mundane carbon filament lightbulb, but it actually has a very interesting origin story. This bulb started its life as a Westinghouse Mazda bulb with a tungsten cage filament. It was sold and used until it burned out. It was then picked up by a bulb recycling company in the 1910's or 1920's, which made a hole on top, snapped the glass rod that held the cage filament in place and pulled it out, stuffed a brand new carbon filament inside, carefully attached it to the lead-in wires with carbon paste, melt-welded a glass tube to the hole on top, gave it a new vacuum, melted off the tube to seal off the newly refurbished bulb, then resold it as a carbon filament lightbulb.
I presume such recycling companies could only fit in new carbon filaments through the tiny hole on top. Bulbs like this can be confusing items, since many recycled bulbs are etched with "Mazda", and all Mazda bulbs had tungsten filaments, never carbon filaments. A small glass "button" is visible at the top of the stem, which is all that is left of the glass rod that once held a cage filament in place. It seems that these recycled bulbs were quite popular, as they are not that hard to find on eBay. The tip of this bulb is larger than normal, because the hole made on top needed to be large enough to fit in the tools to attach the new filament in place. This 3 loop filament variant is a little bit harder to find.