On page 133 I report the attendance at the founding meeting of the Socialist Review group, with a list of initials, not all of which can be identified.
John McIlroy tells me that he thinks BC from Sheffield was almost certainly Bill Cleminson.
Cleminson was an engineering worker and member of the RCP who had been critical of Labour Party entry.
Mike Pearn points out that there is a letter from Cleminson in a 1956 issue of Socialist Review. This would seem to confirm the identification of BC as Cleminson.