Back in 1980, Pat Hawker G3VA(SK) spoke about this circuit in his technical topics column for the RSGB Mag. This circuit has surfaced again in the latest issue of SPRAT. I do not have access to the SPRAT article but I do own a copy of the RSGB Technical Topics Scrapbook which comes with a CD of all 50 years of Pat’s column.
Other than that, I do not know all that much about this circuit and I cannot find any other references to it, or the references in the article, though the sprat article does go into its mode of operation. So while I do not know all that much about it, what I do plan to do is to build this circuit on some vero or scrap copper clad board and run it through its paces and see how it performs and then compare that to some known mixers I have here and try and quantify its qualities.
Anyway, i will post more on this once I have done the work and published the results. For now, I am off to LTSpice to simulate it and see what happens.