This lamp was for use in a photographic print enlarger. My dad was a photographer and we had a darkroom in the house. The bulb has a very high quality ceramic finish so that the filament would not show up on the image being enlarged from the negative. That's also why the etch is on the neck of the bulb and not the top. They also had short lives - I think maybe 250 hours. The bulbs burned hot and very white, probably around 3,200K.
I need to expand my incandescent collection, All I usually get those for is my lights in my room.
I wonder where I can get lots of them..