An example of early 1950s American practice in respect of high-performance point-to-point SSB/ISB receivers is provided by the Crosby 155 triple-diversity unit, described in the attached article from Communication Engineering 1953 July-August.
Crosby SSB Diversity Units CE (FM-TV) 195307,08.pdf
It comprised a trio of Crosby SSB adaptors and a Crosby diversity combing unit attached to a trio of Hammarlund SP600JX HF receivers. So in using the HF receiver + adaptor combination, it was broadly similar to the Marconi CRD150/20B + SSR2 setup.
Perhaps unusually for “full-sized” SSB/ISB units, it had electronic AFC via a reactance modulator rather than a motor-driven system. To ensure that the tuning did not revert to a nominal centre position when a received carrier dropped out, the AFC system included a so-called “infinite time constant” that held position when the carrier returned, thus imparting the same behaviour as obtained from motor-drive systems.
Essentially the same setup was available using Collins 51J receivers instead of the Hammarlund SP600JX. This was noted in an item in Tele-Tech 1953 March.
from Tele-Tech 195303 p.100.pdf
Another feature of the Crosby SSB unit was that it also provided for exalted carrier reception of AM (and PM) transmissions. By way of background, Crosby had worked for RCA for many years in both the FM and point-to-point communication fields. For the latter he had developed diversity receiving systems and also exalted carrier systems, the latter the subject of a paper in 1945, after he had left RCA, I think. He established his own company in 1948, which as well as developing systems, also manufactured diversity and exalted carrier adaptors, as well as exalted carrier receiving systems based upon proprietary HF receivers. From there it would have been but a short step to building ISB adaptors, once it was obvious where the future of HF communications lay. That SSB/ISB receivers could be – and were used for exalted carrier reception of AM transmissions essentially made dedicated exalted carrier receiving systems redundant. Crosby may have envisaged wider use of PM at HF, but that didn’t happen per se, although of course SSB is a mixture of AM and PM.
Here are a couple of 1951 Crosby advertisements for firstly, a triple-diversity exalted carrier receiver and secondly, a triple-diversity SSB receiver.
Electronics 195103 p.334 Crosby Exalted Carrier Receiver.pdf Electronics 195112 p.390 Crosby Triple Diversity SSB Diversity Receiver.pdf
Cheers,