The 350B is an audio output beam tetrode with 25 Watts anode dissipation. It is similar to the 6L6G but possibly less rugged. For normal use the anode would be at 250 Volts.
The 350B, 350A and 367 are all essentially the same valve electrically but differ in base connections.
The pinch stem and within the anode can be seen the bright beam plate.
The control grid is wound on copper rods and there are heat sinks on both the top and bottom of the control grid supports. The anode supports have additional ceramic insulators to withstand high voltages.
The top mica showing the control grid heat sink curving round the central cathode. The screen grid supports are in line with the beam plates that bend around the end of the screen grid supports.
The lower mica: the lower control grid heat sink is clearly seen.
The classic envelope is 51 mm in diameter and, excluding the IO base pins, is 117 mm tall.
