1) Often used of single things where the perception is that two or more parts are involved.
1548 a paire of jeate beades, Bishop Burton
1573 ad quoddam forgam vocat’ a paire of Smythies, Honley
1588 one paire of lowmbes, one paire of heldes and one slaye, Dalton
1655 desired a little parcel of land of the waste ... for building a pair of staires, Leeds