Originally, a kit gun was a S&W model 34, a small frame revolver chambered for 22lr or .32. It was intended as a part of an outdoor "kit" that you might want in your tackle box or whatever.
Or, it could be a 15 or 10 with a 80% lower. If the lower is not complete i.e. 80% it is considered a blank and no serial number is required and no fll transfer required. I believe that glock also has a kit pistol, seen one several years back, and believe they are still manufactured.
You could also make a 47 the same way, i.e. Blank piece of sheet metal that requires bent and holes drilled. It would also come in a kit form, if that's what your asking?
If you build your rifle/pistol yourself, it could also be considered a kit even though it's serialized.
Hope this helps.