Michael,
The trade abbreviation MTS&O would be "Machine Tool Setter & Operator", which was one of the more skilled engineering trades (no doubt Trade Group I), and would indicate what many would term a machinist (setting up and operating lathes, boring, drilling machines etc.) One would expect most MTS&Os to be employed in major engineering depots, station workshops, etc, rather than in small units which would not be equipped with this type of machinery, although even squadrons would probably have at least one MTS&O on strength.
David D
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