The EF39 was described by Mullard as a variable-mu RF pentode, with sliding screen characteristics, for use as a controlled RF or IF amplifier. The EF9 is electrically identical to the EF39 but is on a Ct8 base. The frequency range of operation would be up to 30 MHz.
The Cossor OM6 is directly equal.
The classic envelope is 29 mm in diameter and excluding the IO base pins it is 84 mm long.
References: Data-sheet, 1040 & 1043. Type EF39 was first introduced in 1939. See also 1939 adverts.