The story
Shortened from a longer, bureaucratic US military designation (an Air Force Specialty Code), where '7' was code for "Craftsman".
Etymology from Wiktionary, CC BY-SA 4.0.
seven-level
Today it means: A person who has qualified for "Craftsman" level of skill in the United States Air Force, with more training than "Journeyman" (five-level), but less than a "Superintendent" (nine-level) Full definition →