The Type 51 is a variable-μ RF tetrode and was made in the USA. The 2.5 Volt heater was the standard for AC valves until the 6.3 Volt standard emerged in the mid 1930's. The Type 35 is the same valve.
For cathode bias the cathode resistor should be 330Ω.
The image shows the outer screen, cylindrical anode and close wound screen grid. Above the pinch the heater insulation can be seen as it enters the cathode tube.
The top cap with control grid connection.
The balloon envelope is 44 mm in diameter, and excluding the UY base pins is 113 mm tall.
Reference: Data-sheet & 1043. Type 51 was first introduced in 1936. See also1936 adverts.