Jno is a diminutive for John.

Biographical information from <<https://my.strathspey.org/dd/issue/3312/|issue 3312>>:  
John Johnson was an 18th century English music publisher, instrument maker and seller based in London. His business was founded in 1740, it is suspected he acquired at least some of the stock of Daniel Wright and Benjamin Cooke (publisher), both of whose publications he reprinted from the originals. Johnson came into prominence just as John Walsh was fading as the preeminent English publisher, and thus was able to publish works of Arne, Geminiani, and Scarlatti. He dated a large quantity of his publications, and the quality of engraving was high. Some of his more popular issues were volumes of country dances, some of which appeared in yearly sets.

After about 1762, when it is presumed that John Johnson died, the imprints are mostly that of a “Mrs. Johnson” or “R. Johnson”, his widow.