The double plate (called a spring plate) was used in the late K5 and all of the K6, and all subsequent CB750 SOHC4 engines. Many riders (back in the day) eschewed the spring plate (because it slips under heavy throttle or sudden clutch drop, by design) and replaced it with the GL1000/1100 base plate instead, which is nearly the same thickness, but solid steel.
The top friction plate has wider fingers (outer edge) than the other plates. This was originally a slant-cut-cork plate, to help encourage the slip at startup. These are usually gone now, having square-cut corks instead.
If the clutch basket has been replaced with a K3/4 or early K5 type (some were, see above comment), then the plates all have the same outer finger width and the special top plate won't fit into the hub. If the rider was real creative, he could also install the K0/1 type basket, with its solid steel hub center, for a tighter tolerance and more solid 'feel' to the clutch - but this type uses just 6 friction plates and a thicker bottom steel plate, so it is usually obvious.
