Later 392 K7 pistons to get some extra CR?
Looks like it. That was a "hop up mod" in 1980. 
Today you can take the CB750 DOHC stock-bore pistons and cut a 1/2" flat around their outer edge (of the crown) to clear the valves, and net a 9.6:1 CR in the early heads (with a 1.0mm overbore, first).
Are you saying the dohc pistons into the sohc? Are you doing this mod? I didn't see it on the website.
Yep, have done a couple of them, have one set of pistons still left from that "run". The stock-size bore of the DOHC engine is 1.0mm overbore in our SOHC engines (62mm) and the wristpinis the same size. The valves can touch the pistons, though, unless you relieve the new pistons -0.5mm for a band of 13mm (1/2") around the outside of the tops. Then it leaves a penta-roof piston in a hemi head, which cuts down on the overlap breathing a little bit. This leads to higher compression with a crisp low-end torque, but requires premium gas be used because of the high compression and lack of swirl-charge action (the middle ridge of the piston breaks up the swirl).
I was doing these back before CruisinImage started making the post-1975 pistons for us. Now you can buy a set of his pistons for less than the machine work on the ones I have!
