The EF184 is a purpose designed high gain frame grid pentode. The EF184 superseded the ubiquitous EF80. The mutual conductance is 15.6 and the closely related (pin compatible) variable μ pentode EF183 has a μ of 12.5.
In 1969 Mullard describe the valve as: A frame-grid sharp cut-off pentode for use as an IF amplifier in television receivers. This implies a relatively wide band function at about 33 MHz. The high mutual conductance was important for wide band width amplifiers. Mullard quote operation at 40 MHz.
The thin glass tube envelope is 20 mm in diameter and excluding B9A base pins, is 51 mm long. The electrode assembly is 13 mm top to bottom.
References: Data-sheet, 3002 & 1040. Type EF184 was first introduced in 1960. See also 1960 adverts.