Whilst this preparatory sketch formed part of a collection retained by Marc Brunel and related to the construction of the Thames Tunnel (1825-43), the exact object depicted in it is unknown.
It may represent a load distribution plate, or have been used to support some sort of crane. In the latter case, it may then be connected with removal of spoil in the Thames Tunnel, or else the installation of brickwork to line newly-dug sections.
It is equally unclear to whom the sketch should be attributed.