The plane underwent many modifications during its production by Stanley. Then, probably during the mid-1890's, a mechanism to close the throat's opening was added so that the plane could work narrow surfaces, like grooving the edge of a board, to add lateral stability to the tool.
The first of which was arching the portion of the sole, forward of the cutter, into an open throat configuration. This mechanism, or 'shoe' as Stanley called it, attaches to the round depth gauge rod that slips through the arched area of the main casting and 'closes' the throat to give more of a bearing surface, or sole, on the tool.
And here you thought routers are the stuff of the modern workshop. They've been around much longer than the ab NORMal kind has, but these kind ain't the 'lectrical kind.
These kind are pushed or pulled, and are suited for smoothing the bottom of a groove, mortice, or whatever, which is lower than the general surface of the piece being worked.
They are particularly practical for routing dadoes for shelves, stair stringers or where pieces of hardware are to be recessed into the surface or edge of a board, such as large hinges or lock strikes, etc.