The Precise adder is the naughy boy of adding machines for reasons seen in the pictures below.
Here is a blurry video demonstration (note: audio is delayed several seconds)
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.