Found by TM on the Wemyss brickworks site.

Unidentified brick.

Find location would suggest Scottish origins but….

Possibly ? – Wemyss Brick Co. Works, Methill, Fife c. 1906 t0 1981

 It was found  close to this similar SLW brick.

The Survery of Scottish Brickmarks  – click me  – have recorded the letter ‘W’ as being a product of the Charlestown Brick and Tile Works, Fife c.1869 – 1882 although this mark is recorded as relevant to a common brick and not a fire brick as in this example.