Those are Burns' verses that Sandy gives, as quoted by William Stenhouse in 'Illustrations to SMM', #264, from a letter of 1794 from Burns to Thompson. Ths song was not published until several years later.

Earlier in his note Stenhouse gave what he said were the original verses. He said that Stephen Clarke took down the tune from Mr. Clunie's singing when he (Clarke, not Stenhouse) and Burns were were visiting Mr. Clunie in 1787.