Hi Andy, welcome to the Forum.
You'll find many details of the various signal boxes on the North Lancashire Loop, including several signal box diagrams, in my book "Signal Boxes on Lancashire & Yorkshire Railway Lines: North-East Lancashire" (details here: https://signalbox.org/books/books2000.htm
& here : http://www.hall-royd-junction.co.uk/Hal ... books.html
). I hope you can track down a copy - hopefully your local library should be able to help.
C. 1904 - 1909 the list of boxes on the line was:
Great Harwood Junction
Little Harwood* (closed 1909)
Queen Brick Siding (closed c.1940)
Norden* (2nd box closed c.1919 - 1922)
Great Harwood West
Great Harwood East
Martholme Colliery (closed c.1940)
Padiham Station West [this & next replaced by one box, "Padiham", in 1913]
Padiham Station East
Rose Grove West
(* The original Norden closed and was replaced by another box, also named Norden, located approx. 880 yards to the east, c.1904 - 1907 (details in book). Confusingly both are labelled as SB on the OS map you cite, which also shows Little Harwood SB, although that closed in 1909 [the actual revision
date of the 1912 map was 1909]. This might help explain the unusual number of SBs that is puzzling you! Norden was just a "break section" or "intermediate" box, which was only provided to increase capacity on the line, and Little Harwood was only built to ease acceptance problems at Great Harwood Junction, so they didn't have any associated sidings etc.)
Hope that helps!