a 120j back (left) vs a 120 back (right)

Both are SQ Type (i.e. ISO wheel on top)

The shells are certainly different from the inside. But both use the same type of darkslide.
IMG_20131013_193923.jpg

edit:

The 120 also says "SQ 6x6" on the back, the 120J "SQ 6x4.5"