The Dream Doctor by Arthur Reeve

There it is, cried Craig scarcely able to restrain himself with the keenness of his chase, there it is--the mark like an L. This cartridge bears the one mark, distinct, not possible to have been made by any other pistol in the world. None of the Hep Sings, all with the same make of weapons, none of the gunmen in their employ, could duplicate that mark.