This is the ribbon copy of the carbon copy at record 3953. The ribbon copy has a postscript in BR's hand: "P.S. Please give kindest regards to Lili Kraus if you happen to be communicating with her". [A Mozart and Beethoven specialist, pianist Lili Kraus played at BR's 90th birthday celebration at Royal Festival Hall, London, May 19, 1962. The programme ad is here. She must have impressed BR, although they didn't correspond.] In his letter (record 3952) Blum had told BR that Kraus was a personal friend of him and his wife.

Re BR's mention of a concert in honour of Schweitzer's 90th birthday, BR had provided a message for the Esterhazy Orchestra for the occasion (see B&R B176).

The letter was advertised for sale in fall 2014 and reported sold at https://www.schubertiademusic.com/lots/index/page:9/catalog:35 with the accompanying text. It is now in the Dr. David Harley Collection (26 Nov. 2018).

164. [Literature & Art] Russell, Bertrand. (1872-1970). 
Signed Letter mentioning Albert Schweitzer and Lili Kraus
TLS, signed "Bertrand Russell" from the British philosopher, logician, mathematician, historian, social critic and political activist. 1 page, 28th Feburary, 1966. Addressed to David Blum, the symphonic conductor and writer on classical music, discussing his hopes to meet later that year in England, noting that he was "very pleased to hear that the Schweitzer concert was such a success," and adding an autograph postscript in which he asks Blum to "please give kindest regards to Lili Kraus if you happen to be communicating with her." 

David Blum made a number of recordings, including works by Mozart and Telemann and a series of Haydn symphonies for the Vanguard label, with the Esterhazy Orchestra of New York, which he founded in 1961 and directed until 1969. He was music director of the Lausanne Symphony Orchestra in Switzerland from 1973 to 1982 and of the Geneva Symphony Orchestra, which he also founded, from 1977 to 1986, and conducted many other orchestras as a guest.
Mr. Blum devoted his later years to writing, showing a particular fascination with musical performers and their approaches to their art. In addition to three books -- ''Casals and the Art of Interpretation,'' ''The Art of Quartet Playing: The Guarneri Quartet in Conversation With David Blum'' and ''Paul Tortelier'' -- he wrote articles for The New Yorker, The Musical Times, The Strad and the Arts and Leisure section of The New York Times.