Hi Joe, the Singer 115 is just an older version of the model 15. It lacks the stitch length regulator plate that you'd see on a 15-91, for instance, and features just a silver knob that you rotate (and guess at) to adjust the stitch length. The 115 also lacks reverse, features a bobbin winding assembly that you have to manually press with your finger while the bobbin is winding, and has the cool, larger spoked handwheel. Cosmetically the decals are a bit more garish (or beautiful, depending on your point of view) than a later 15, and you may find the faceplace is completely smooth, rather than having a textured pattern.
"CB shuttle" stands for Central Bobbin, and from what I understand this term was used to distinguish it from the older pointy shuttle system that it eventually replaced; but I can't really back that up or elaborate on this, as I'm parroting what I'd heard or read someplace else. I bet the Vintage Singers Yahoo Group will have more info on the "CB" distinction.
hope that helps,