Gun Sight, Reflector, Mk IIIA 7A/1313 RAF A-20 The knob is used to

The knob is used to adjust the altimeter to the barometric pressure as displayed in the small window at the 3 o'clock position

Pulling and turning would adjust the Nose Down marker

This version was made by General Electric Part Number 8DJ17AAT

and includes descriptions

Gun Sight, Reflector, Mk IIIA 7A/1313 RAF A-20 The knob is used toHere is a WWII era Reflector Gun Sight Mk IIIA, 7A 1313. The Mk IIIA was adopted by the RAF as the standard turret and free gun sight for their aircraft. The Mk IIIA consisted of a detachable lamp unit and an upright sight head. The lamp unit is removed by depressing a small button on the bottom of the housing, revealing the bulb and circuitry (this example is quite tight). The brightness of the bulb was adjusted by the gunner by rotating the rheostat