do not believe that is what the OP is asking about. Back in-da-day the ratings were net HP. Today they test the engines with all the accessories and exhaust in place so the numbers are less than if tested with nothing extra on the engine and using dyno headers.
But on to the OP. YES, and it is still done today. That is one reason my Bonneville that weighs more and has less rated power than my buddies Honda still blows away his Honda. My Buick was rated at 235 HP------at 4,000 RPM’s. At 5,300 RMP’s it is more like 300+.
Of course the area under the torque curve is what really tells the story.