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 Mazda bulb with a tungsten cage filament. It was sold and used until it burned out. Apparently, it was a 40 watt, 115 volt bulb (some of the lettering on top is still there. In fact, this bulb used to be a tipless bulb! As evidenced by the presence of an evacuation port in the stem). It was then picked up by a bulb recycling company in the 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. Lightbulbs began to lose their vacuum tips starting in 1922, but this bulb proves that some tipped bulbs were still being made after that (out of dead tipless bulbs, no less). 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 stub 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 bulb really belongs in the vintage gallery, but I put it in the antique gallery so it could be together with the 3 loop fil. recycled bulb.