Memories are made of this.
The first draft of rules said the power meter was to be calibrated using the "standard charge" of .45 ACP for major, 9mm for minor. In another bulletin Cooper noted the Star PD (3.9".barrel) and commercial match hardball was what they used at API.
In our club it was the practice to calibrate major loads using a Colt Commander (4 1/4" barrel) and Winchester 230-gr. FMJ. I was the only club member who owned such a pistol, actually a satin-nickel Combat Commander I had picked up secondhand. In retrospect, it was probably the main reason they let me join.
A couple of bullet dents on the frame of the power meter resulted from another problem. We found a lot of shooters couldn't shoot accurately enough to hit the 3" plate from about six yards. Or if a good shot did all the testing, he'd find pistols not sighted in well enough to hit the plate.
The power meter wasn't very precise, but it did serve the goal of ensuring competitors were using full power loads.