This unusual bulb is what I'll call a "transitional" bulb. The shape of this bulb was originally designed for an uncoiled tungsten cage filament, with a glass envelope with flat sides and plenty of space to fit a long cage filament inside. However, coiled tungsten filaments are very compact and don't need all of this excess space. Ideally, the space between the filament and any part of the bulb would be uniformly even, just like in most incandescent bulbs. But, for some reason, the maker of this bulb used the old shape for cage filaments, for a small coiled filament. Could this bulb have been made in a brief point in time where the manufacturer already had everything in place to make coiled filaments, but just didn't have the molds for the new bulb envelope shape? Or maybe they had a large overstock of flat-sided bulb envelopes and they just wanted to get rid of them without wasting them. There is a lot of empty space on the upper part of the bulb. I also noticed that they seemed to have made a compromise by making the stem extra long.

These "transitional" bulbs aren't likely to be worth a lot of money to a collector even when they are extremely rare. Still, I think it's an odd & interesting item to have.