5WS series stoves require 2 rear firebricks, they are listed as LHS (left hand side) & RHS (right hand side). Please note this is not as you look at the stove.
12 series stoves requires 2 x rear bricks
Additional information
Stove
4 series, 5 series, 5WS series LHS, 5WS series RHS, 6 series, 8 series, 8WS series LHS, 8WS series RHS, 10 series, 12 series