This pair of Mullard advertisements from WW 1948 illustrate both the overlap of and the differences between the EF42 and EF91.
The EF42 was presented as a TV frequency changer. The EF91 was presented as being suitable for several applications in TV, FM and VHF communications receivers, including RF amplifier, mixer, oscillator and IF amplifier. The mixer and oscillator example circuit shown was very similar to that shown for the EF42.
These 1954 Mullard advertisements for the EF80 and EF91 show their similarities more than their differences.
The EF80 was described as being primarily a RF or IF amplifier for TV receivers, but also suitable for use as a mixer, video amplifier or sync separator.
The EF91 was described as being primarily a RF amplifier or mixer for TV receivers. This was rather a narrow definition of its application range, but then Mullard did sometimes tailor its advertising to show specific rather than all applications of a given valve. (The ECC81 provides a good example of this.)
Be that as it may, advertising a high-slope pentode as a TV RF amplifier in late 1954 was rather anachronistic, as by then rest-of-world best practice had moved to the cascode as the choice for this job, and the UK was on the cusp of catching up with change.
The EF80 was supplemented, but not supplanted by the frame-grid EF184 in 1959. The EF184 also appears to have been the industrial offering in this class, so it also could be seen as supplementing the EF91.
Cheers,