This steel and brass component served as an adder, it connected two shafts, a and b, whose sum c = a + b was to be obtained. The adder consists of a differential gear. The gear ratio was chosen so that the sum of the revolutions of shafts a and b would equal a revolution of shaft c.
Parts of Bush differential analyzer at NMAH have museum numbers MA.314824 and 1983.3002.01 through 1983.3002.89.