The "J" frame is the Smith & Wesson small frame revolver such as the model 36 .38 special. The smallest frame, as in the model 36 Chief's Special.
The grip frame on the X-Frame series interchanges with current K-frame and L-frame guns. In other words, any wooden grip that would fit a current 686 will fit the .500 Magnum.
L frame, blue steel revolver in 357 Magnum
A K frame revolver.
I frame
The model 29 is one.
Medium
It is on the frame.
Frame, barrel and cylinder
Yes, a model 66 is a K frame revolver.
On the underside of the frame
On the frame and the grip frame.