VHF transmitting triode. Thoriated tungsten filament. Grid and plate pins.
50W plate dissipation, 3KV max plate voltage at 150mA max, gain 27. Up to 200MHz max full-ratings operating frequency.
Tantalum plate shows dull cherry red color when operated at 50W.
Heintz & Kaufman, Hytron and even STC, as 3B/501A, known manufacturers