or to conform with manufacturers' model numbers. Recently this rule has been ignored, and aircraft have received a design number equal to the model number (e.g., KC-767A) or have kept the design number when they are transferred from one series to another (e.g., the X-35 became the F-35). Series letter Different versions of the same basic aircraft type are to be delineated using a single letter suffix beginning with "A" and increasing sequentially (skipping "I" and "O" to avoid confusion with the numbers "1" and "0"). It is not clear how much modification is required to merit a new series letter, e.g., the F-16C production run has varied extensively over time. The modification of an aircraft to carry out a new mission does not necessarily require a new suffix (e.g., F-111Cs modified for reconnaissance are designated RF-111C), but often a new letter is assigned (e.g., the UH-60As modified for Search and Rescue missions are designated HH-60G). Some series letters have been skipped to forestall confusion with pre-1962 naval designations; for instance, there was no "H" version of the F-4 Phantom II because the aircraft type was previously designated F4H. Non-systematic aircraft designations Since the 1962 system was introduced there have been several instances of non-systematic aircraft designations and skipping of design numbers. Non-systematic or aberrant designations The most common changes are to use a number from another series, or some other choice, rather than the next available number (117, 767, 71). Another is to change the order of the letters or use new acronym based letters (e.g. SR) rather than existing ones. Non-systematic designations are both official and correct, since the DOD has final authority to approve such designations. RC-7B Designation conflicted with unrelated C-7 Caribou (later redesignated EO-5C in August 2004). F/A-18 Hornet, also the transient F/A-16 and F/A-22. Originally, the Navy planned to have two variants of the Hornet: the F-18 fighter and A-18 light attack aircraft. During development, "F/A-18" was used as a shorthand to refer to both variants. When the Navy decided to develop a single aircraft able to perform both missions, the "F/A" appellation stuck despite the designation system not allowing for slashes or other characters. AF-18 would be conformant. Similar issues existed with the naming of the F-22, though the naming of an attack variant was mooted by designation of the FB-22 (which, more appropriately, should have been designated BF-22). F-35 Lightning II The