It looks like the last 5-1/2" were cut off the frame, based on the 6400 I have here.
My guess is that the lift and bed weren't made for that model truck. When it was mounted with the pivot at the back of the frame, there was too big a space between the back of the cab and the headboard. So they moved everything forward and cut the tail of the frame off to allow tilt without hitting.
It looks a frame mount not a sub-frame.
How long is the bed? It looks a little long. 12' would be normal I think.
