Fairchild Republic—A-10 Thunderbolt II: USAF Variants and Serials YA-10A: The two prototype Thunderbolt IIs were assigned the designation YA-10A as they undertook service tests with both the manufacturer and Air Force Systems Command. Six pre-production aircraft were also allocated this designation. · 71-1369 to 71-1370; 73-1664 to 73-1669 OA-10A: The prefix O was applied to numerous A-10As which were redesignated for Forward Air Control (FAC) duties. The OA-10A refers to the mission as no external modification was carried out and there are no visible means to distinguish this version from the basic Thunderbolt II. · Dozens of A-10As redesignated without modification YA-10B: The first pre-production A-10A was converted to YA-10B configuration as the proposed two-seat trainer version. Following an evaluation by the manufacturer and the Air Force, the project was abandoned and the sole YA-10B placed on display at Edwards AFB. · 73-1664 A-10B: The designation was assigned to the production versions of the two-seat Thunderbolt II, although none was built after evaluating the prototype. GYA-10A/GA-10A: The prefix G, indicating ground instructional airframe, was assigned to the fourth and fifth aircraft built following their transfer to the Sheppard Technical Training Center and to a pair of ex-TAC A-10As employed by ATC TTC. · 73-1665; 73-1666 (GY); 75-0260; 76-0518 (G)