The "ZR" Tail markings
Contrary to popular belief, the ZR embellished on the tail of the 26 TRW's RF-4Cs does not stand for "Zweibrucken Reconnaissance". To fully understand the assignment of the "ZR" tail code on 26 TRW aircraft, a little background information is appropriate. Pacific Air Forces (PACAF) began the use of tail codes in late 1966. This practice was adopted by the Tactical Air Command (TAC) in early 1968 and was subsequently extended into USAFE in 1970.
The first letter of the two-letter code represents the wing, and was usually the first letter of the base's name. The second, which designated the squadron, consisted of a series of letters which were the same for all USAFE units with the exception of one. The letters R, S, T, U and V were used by USAFE units. The one exception was Torrejon Air Base in Spain, where the 401st Tactical Fighter Wing used the letters J, K and L since the "TS" and "TT" codes were used by two squadrons of the 56th Special Operations Wing at Nankhon Phanom Royal Thailand Air Force Base.
On 1 April 1972, the system was changed with the implementation of Air Force Regulation 66-1. A single code was used to designate an entire USAFE wing rather than one for each assigned flying squadron. As a result, many of the codes were eliminated, along with the introduction of many newer ones. The 26 TRW "ZR" markings were adopted when the unit transferred to Zweibrucken on 31 January 1973.