The CV2815 (commercially the ML4) although listed as a triode output valve, the ML4 was the definitive descendent of the famous (or infamous) Type KL1 of 1926 and was primarily a professional and military type, little used in domestic radios.
It was introduced in 1929 and an improved version appeared two years later. The original ML4 was made with a balloon envelope.
It was quite widely used as an RF oscillator, as a single-valve phase splitter, and as a driver for larger valves.
The shape matches the Osram ML4 and this unmarked valve was probably made by M-OV.
Looking up from the base reveals a rectangular cathode the the heater passing through the tube four times.
The dome and top mica. The grid supports show the notches and thus the grid pitch.
The classic envelope is 45 mm in diameter and, excluding the B5 base pins, is 99 mm tall.
