The numbering runs sequentially from 1 (March 1953) to 188 (December 1972) at which point David and Charles appear to have bought the Readers Union from Dent and absorbed all the old titles left in stock (including 'optionals') renumbering them alphabetically by title from 5001 "Age of Pussyfoot, The" to 5067 "Worlds of Robert F Young, The", and then numbering their new books from 5068 ("Solaris") in January 1973. (See Fourth column in the table below.)

About November or (December) 1980 by which time we've reached 5173 (or 5172 - problem here being not only are these two numbered out of sequence but 5172 "Transfigurations" is actually the Gollancz edition as indeed 5161 "The Fellowship of the Talisman" had been Sidgwick & Jackson's - ditto 5176 and 5181 later).

Another complication with 5173 "The Priests of PSI" is that the inside flap of the dj calls this "New Series: Vol 1 - issue 13". From here on in they continue with this vol/issue number. (See Sixth column below.)

The final book I've been able to find is "The Winter Plain" (albeit this is Vol 3 - issue 12 and so is "King David's Spaceship"!). I just don't know if there were any further books from here - I've certainly never found them.

William Seabrook

The notes above are taken from email correspondence with William Seabrook in 2000. The numbering of the later SFBC editions (post issue number 170, which is the last actual numbered issue) is bizarre bordering on the incomprehensible. There is in fact NO numbering of any kind on either the books or dustjackets and any idea of a numbered series can only be seen by reference to the flyers such as SFBC NEWS. However, the numbering (which is just stock numbers in fact)  is sometimes out of sequence, and at least once duplicated. It's a shambles, really.
Any further information gratefully received - contact me! - Greg Pickersgill