Complex sentence: The remaining aircraft was a modified C.1 converted for photo-reconnaissance, the Andover C.1 (PR), serial number "XS596"; the UK-designated aircraft under the Treaty on Open Skies.
Split into simpler sentences: The remaining aircraft is a modified C.1 converted for photo-reconnaissance, the Andover C.1 (PR), serial number "XS596". This is the UK-designated aircraft under the Treaty on Open Skies.