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I'm a little confused about the manufacturer designation list. I've seen two different designations used for Allison engines (e.g., T56-A-427 and T406-AD-400) and for Pratt & Whitney engines (e.g., T73-P-1 and F100-PW-100). What is the explanation for these? D271l (talk) 05:49, 4 July 2021 (UTC)[reply]
Good question. The simple explanation is that under the original system used, both single and double letters were used for manufacturers, but only double letters are used under the current system. See http://www.designation-systems.net/usmilav/engines.html for further explanation. I'm not sure if there was a period of overlap somewhere or not, but it looks like the primary change-over date was 1968. BilCat (talk) 06:31, 4 July 2021 (UTC)[reply]
