My theory is a little different: The 65% (230 bhp) column lists two airspeeds, best power, best economy. Obviously 230bhp is 230bhp no matter where the mixture is, the airspeed should be the same for a constant power output.
So only one of these columns is correct.
My guess is the best power column is and refers to 230bhp, and the much lower fuel flow (30.8gph) and slower airspeed is an engine actually producing about 210 BHP, or 60% power, LOP. Would explain the speed drop, and they'd have got these numbers from flight testing? It was the 1970s, almost the apogee of piston engine management ignorance. So maybe whoever was doing the flight testing had forgivably poor grasp of LOP procedure and neglected to add the 2" or so manifold pressure (in fear of frying some valves, Oggers), like what the FE's of yesteryear did with their BMEP gauges and whatnot.
"...the single drive, dual mags (J2BD)...". What a bastard idea that is.
Apart from the obvious single point of failure bit, why else are these bastards? I've heard of a failure in a Mooney or something where the whole unit moved a bit and timing got so far out the aircraft forced landed. Anything else?