I do not recall ever having seen such an item. So it probably never was a large volume lamp (bulb). That makes it more likely to be discontinued. The addition of a dimmer to the cord or to the lamp assembly would help to give varying light levels.
What are the wattages of the lamp you have? The brass screw base may have it stamped in tiny numbers/letters.
My Internet search found no source for the lamps.

The ring contact for the second filament on the bulb base is going to have very tight tolerances for 120 volt bulbs.

Are you sure this is not a low voltage DC bub?