Correct, the frame is wider on a 72+ and the mounts at the ends are different. There's 2 styles of 68-71 bar, one is hollow stamped metal box and the other is a rectangular tube with brackets welded at each end and in the middle, they both interchange. The 72+ type 4 bar has no brackets on it anywhere, the 72+ 1600 bar has brackets in the middle but the outer ends mount to the frame in a completely different way.
Most common 68-71 (disregard the added brackets bolted to the mount rubbers) :
Less common but still fits (better exhaust clearance too) :
