That protrusion is a brass stylus, conveniently stored in the undercarriage. It is used to engage the gear pins to dial the number desired, which is added to the result seen in the window.
An unusual aspect of the the Precise adder is the glass lens that sits across the result window and serves to magnify the digits seen there.
The handle on the right clears the result to zero.
Here is the drawing for the patent referenced on the machine's front plate
Patent 2402549 O. R. Hartom 1946