The dull emitter FER2 is an RF triode and was made in two versions. A four Volt filament and a six Volt 0.1 A filament. The details of precise type would have been on the missing paper label. The FER1 exhibit retains the paper label and shows the form it took.
The box says that each valve is stamped with the BBC stamp and here it is in red and upside down.
The identification of it being the RF type comes from the blue paint on the top around the pip.
The balloon envelope is 46 mm in diameter, and excluding the base pins is 99 mm tall.
Reference: 1043. Type FER2 was first introduced in 1925.