The SG Junior was the basic no bells and whistles SG. It came equipped with just one single coil P90 pickup, in the bridge position, but was otherwise the same as the SG Special. It was priced at $142.50 at it's launch in June 1961, and like the SG Custom and SG Standard was initially named as part of the redesigned Gibson Les Paul range. See the Les Paul Junior from which it descended here.
A best seller that's even better - with it's handsome cherry-red finish and it's highly desireable new double cutaway sculptured design, and it's very popular price. You get Gibson quality and top performance
It sold well, being the biggest selling SG guitar of the 1960s, and the second best selling solid-body after the Melody Maker