I have found a little more on the Marconi CR150 series receivers.
The CR150-2 had frequency coverage of 1.5 to 22 MHz in 4 bands. It was mentioned a Practical Wireless 1967 October article on the CR100 and CR150, p.414ff. I’d guess that this variant would have introduced the 1.2 MHz 1st IF, as1.6 MHz would not have worked so well for a receiver that tuned down to 1.6 MHz. In the case of the CR150, the 1.6 MHz 1st IF was 80% of the lowest received frequency, namely 2.0 MHz. It would that the same proportion was retained for the CR150/2, 1.2 MHz being 80% of 1.5 MHz.
The CR150/5 was mentioned in connection with an enhanced radio equipment fitout for the vessel SS Gothic in anticipation of its use for a Royal Tour, see:
http://maritimeradio.org/ship-statio...or-royal-tour/, which includes an excerpt from the Marconi Mariner 1952 January-February. Noted was that a CR150/3 was installed for traffic purposes, with a CR150/5 used for cuing in respect of BBC commentaries. Thus we can deduce that the CR150/5 was available by the beginning of 1952, but that the CR150/6 (successor to the CR150/3) might not have been, hence the use of the CR150/3. Why two different CR150 variants were used is unknown. Different frequency coverage requirements was a possibility. The CR150/5 is at least a suspect for being the “unknown” variant described in R&TVERB. Essentially the same information about the Gothic installation was provided in an article in PW 1954 January, p.24ff. Interesting was that the installation was a joint effort between MIMCO and MW&T.
Judging by this item -
http://www.nas.gov.sg/archivesonline...b-001a4a5ba61b - the CR150/6 was in place by the beginning of 1954, though, and it was associated with the HU12 double diversity telegraph receiving unit. This was the Group 3 telegraph equipment mentioned in the EE 1954 June article, intended for use with regular communications receivers. It seems reasonable that the CR150/6 would have been Marconi’s default receiver offering for this purpose.
During the valve era, the range of receivers within Marconi’s Group 3 point-to-point category appears to have been progressively extended. The HR100 was a rebadged Eddystone 750. The HR110 was an Eddystone 910 variant. And the H2301 was an Eddystone 880 variant with a choice of 3 or 6 kHz SSB filters and a crystal-controlled carrier insertion oscillator. The HR120 was an own-design high-stability receiver of the “converter” type with a crystal-controlled first oscillator, decade switch tuned. One imagines that it was intended to offer frequency-agile SSB capability with better than VFO stability and perhaps approaching that provided by the spot frequency crystal option in the HR22. It does not appear to have been fitted for pilot carrier recovery, though. The HR120 appeared in 1960. That was after Racal had introduced outboard SSB/ISB adaptors (and telegraph adaptors) to work with its RA17 HF receiver, taking advantage of the good stability of the latter at all frequencies.
Cheers,