Sentence:  >>> A 'tweaker' being a very small screwdriver. I first met that term in 1955./  >>We called them <WSD>7-Levels</WSD> in ADC./  > Why?/ A 5-level is a term for a journeyman technician (3-Levels are trainees), and a 7-level was management-expert technician./ I'm not sure of the history, but it went something like the only difference between a 5-level and a 7-level was the size of his screwdriver. 
Meaning of the word marked with <WSD> tags:  Short for seven-level screwdriver, so-called because it was standard seven-level equipment.