Additional heating was expected with the hypersonic speeds of the X-15A-2. As a result, most of the structure was covered with an ablative coating. The shutter covered the left window to protect it when the coating burned away - coating the windows with the burnt residue. After the high speed portion of the flight ended, the shutter would open and the window would be clear for the pilot to see out of.
North American
X-15A-2
The X-15A-2 was modified from the number two aircraft and delivered to NASA in February 1964, It included among other new features, a 28-in. fuselage extension to carry liquid hydrogen for a supersonic combustion ramjet that was flown (as a dummy) but never tested. It also had external tanks for liquid ammonia and liquid oxygen.