1) In connection with tanning the OED defines this as ‘the thickest and best-tanned hide’.
That meaning seems implicit in the first example quoted there, from the Act of 1535: ‘or any leather called backs or sole leather’. Yorkshire references date from an earlier period: c.1423 what man as wyrkys a dakyr of backs, York
1541 ij bakkes and ij dyntles xiijs, Knaresborough
1673 In the Tan Yard, 39 uper leather hides, 18 backs and halfe att 15s pece, one with another, Ł43 2 6, Selby. The back is actually the main portion of the hide and a Latin reference in the ordinances of the cordwainers of York provides earlier evidence of its use: 1417 si invenerint aliquod corium nigrum non tannatum, York.