This is a multi filament valve made by the Nelson Valve Co.
in the mid 1920’s. At that time valves were very expensive to replace when the filaments went open circuit. Some manufacturers offered a repair service. They cut open the valve’s glass envelope, replaced the filament, re-fused the glass and evacuated the envelope again. Nelson overcame the
filament o/c problem by installing three separate filaments mounted side by side. Two brass stud switches mounted on the base of the valve could be used to bring any filament into use. Also for power valve operation two filaments could be used together still leaving one as a spare.