My further research after being tipped off by Ian Hardie found the source of what we might call the "striking error" that others have copied without due citation to him is W. T. Calman (1912, pp. 451- 457) who was shown the three letters on or before 1912 by Matthew's daughter Euphemia. Claman (1912 p. 451) wrote that the letters had never before been published. Matthew and Darwin scholars should note that the above letter is one of the three later originally found by Ian Hardie and Min Hunter at the home of John Matthew after his death. They set up the Patrick Matthew Trust, which employed Jim Dempster to research and write on this topic.
Using a secondary transcribed source such as Calman's published transcription of 1912 yet citing the material as though it is the primary source whist ignorantly replicating a transcription error unique to the secondary source is one of the most simple forms of poor scholarship counting as academic misconduct by plagiarism. This mistranscription and wrong attribution issue on the Darwin Correspondence website and elsewhere in the literature on Darwin and Matthew is just one more ironic example of the many issues of dreadful scholarship plagiarism that that run through the so called "Darwin industry" like hidden woodworm in an antique.
Note: For the historical record, I have a copy of van Wyhe's Sutton's research on Darwin and Matthew is a "conspiracy theory' email that he sent to a Scottish journalist. But for reasons of his copyright, I cannot legally publish it.