Found by TM on the Wemyss brickworks site.

Unidentified brick.

Find location would suggest Scottish origins but….

It was found close to this  W Brick. There were a number of these cladding a wall. They are slim about 1″ wide.

The SL looks to have been added separately to the W.

Possibly ? – Wemyss Brick Co. Works, Methill, FifE.