This brainteasing benefactor presents us with an enigma. Searching the web we found a book "Heretical Doctrines of the Plymouth Brethren by one unknown - yet well known" published 1852. To quote Wikipedia: "the Plymouth Brethren is a conservative, Evangelical Christian movement, whose history can be traced to Dublin, Ireland, in the late 1820s." The Whitechapel drinking fountain was first erected in 1860 so its mystery funder could be the same person as the retiring author; they are certainly both interested in religious matters. But then so were a lot of people at the time and the riddle "one unknown yet well known" may have been in common usage. Does anyone have any answers?
2022: We are grateful to Frances Pyne who responded to our plea with "This might or might not help your puzzle of 'as one unknown, yet well known.' That is a direct quote (depending on the translation you are using) from 2 Corinthians Chapter 6 verse 9. It is used to refer to how the faithful should view themselves and strive to be viewed: As servants of God, we commend ourselves . . . as servants of God, we commend ourselves. So, although not getting to a specific person, it might help to explain how the writer or, more likely, writers wanted to be viewed; they were demonstrating that they were part of the 'body' one of the faithful."