This indicator was installed on the pilots instrument panel of US military aircraft designed with the Low Altitude Bombing System (LABS) used for "Toss-Bombing", or "Over-the-Shoulder" bombing, also known fondly by pilots as "the idiot's loop".
From our friends at Wikipedia:
Toss bombing (sometimes known as loft bombing, and by the USAF as the Low Altitude Bombing System, LABS) is a method of bombing where the attacking aircraft pulls upward when releasing its bomb load, giving the bomb additional time of flight by starting its ballistic path with an upward vector. (see diagram in photo gallery)
The purpose of toss bombing is to compensate for the gravity drop of the bomb in flight, and allow an aircraft to bomb a target without flying directly over it. This is in order to avoid overflying a heavily defended target, or in order to distance the attacking aircraft from the blast effects of a nuclear (or conventional) bomb.
The LABS system was installed in versions of the F-86 Sabrejet, the F-100 Super Sabre (see photo from F-100C Flight Manual, item 16 on the panel), and the B-57 Canberra. Measures 3.25 inches diameter and ~2inches deep. It is in exceptional condition.