Mechanic was also used as a kind of catch-all term for the working class, as in Mechanics' Institutes.
Interestingly, all the men in that extract are listed as Mechanics, apart from one Farmer, and all the single women are "Spin" - Spinners or Spinsters? Unless this was a bulk emigration from a cloth-making area, I suspect Spinsters. This suggests that unless someone had a particular skill or trade, they may have been using "Mech" and "Spin" as general terms for men and women of working age.