Some numbers were skipped when a number was requested and/or assigned to a project but the aircraft was never built. The following table lists design numbers in the 1962 system which have been skipped. *: Skipped to avoid confusion with the North American T-2 Buckeye, which was still in service at the time. **: The T-4 and T-5 designations were skipped in favor of T-6 by Raytheon to honor the WW2-era North American T-6 Texan. The designations are considered skipped as the sequence continued with the T-7 rather than continuing from the last otherwise sequential designation. #: A-8 was technically skipped, but the AV-8 Harrier received the number within the "V" vehicle type sequence. Within the V sequence, it should have been designated AV-12 (as the Ryan XV-8 "Fleep" was already in existence). †: The C-42 through C-44 designations were skipped in favor of the C-45 by Airbus. The designations are considered skipped as the sequence continued in with the C-46 rather than continuing from the last otherwise sequential designation. Manufacturer's code From 1939, a 2-letter manufacturer's code was added to designations to easily identify the manufacturer and the production plant. For example, F-15E-50-MC, the "MC" being the code for the McDonnell Douglas plant at St. Louis, Missouri. Block number In 1941 block numbers were added to designations to show minor equipment variations between production blocks. The block number appears in the designation between the model suffix and manufacturers code (for example F-100D-85-NH). Initially they incremented in numerical order −1, −2, −3 but this was changed to −1, −5, −10, −15 in increments of five. The gaps in the block numbers could be used for post-delivery modifications, for example a F-100D-85-NH could be modified in the field to F-100D-86-NH. Not all types have used block numbers. See also British military aircraft designation systems Hull classification symbol Italian Armed Forces aircraft designation system Japanese military aircraft designation systems List of military aircraft of the United States List of United States Navy aircraft designations (pre-1962) RLM aircraft designation system Soviet Union military aircraft designation systems United States military aircraft designation systems Explanatory notes References Citations General and cited references External links U.S. Military Aviation on Designation-Systems.net by Andreas Parsch—a reference source with details, examples and history Non-standard designations page on Designation-Systems.net Designation Systems FAQ on hazegray.org by Emmanuel Gustin U.S. Military Aircraft Designations 1911–2004 on driko.org by Andrew Chorney U.S. Military Aircraft