Each piece is referenced with a letter and number. Eg W285 Sometimes there is also a letter after the number. Eg W289B.
The first letter relates to where the piece was made:
S = Stourbridge
K = Royal College of Art
P = Peterborough
W = Isle of Wight
F = Farnham
The number is the firing reference (sequential) and if there is a letter at the end of the reference it relates to the shelf the piece was placed on in the kiln. A was always the top shelf.
Usually a number of things were fired in the same kiln, taking advantage of differing temperatures within the kiln.