Here is something I have wondered about "Rawalpindi Blues", Track 12 on this Blindfold Test. In the booklet that comes with the LP set of Escalator Over the Hill, the lyrics are printed in a back section. The lines that Jack Bruce sings on "Rawalpindi Blues" are credited to "Jack". There are responses sung by someone else, to what Jack is singing, credited to "His Friends." For example, :"His Friends" sing "let me stay away from you," and "what will we ever do with you."
There is a separate section in the front of the booklet with the title "Musicians." . It shows that Jack's Traveling Band, the band on "Rawalpindi Blues", consists of John McLaughlin, Carla Bley, Jack Bruce, and Paul Motian. There is no mention of "His Friends" in that musician credits section.
There is yet another section near the front of the booklet with the title "Cast.", That section lists characters' names and which musicians and singers play them. It does not say which songs they appear on. In the "Cast" section, "His Friends" are Charlie Haden and Steve Ferguson.
So is it Charlie Haden and Steve Ferguson singing the response parts on "Rawalpindi Blues"/ I have read that Charlie Haden was part of a family country band as a child. I am not sure if Charlie Haden could sing well.
It is not made certain and clear, in the booklet and its credits.