What are the first two serial numbers of your Parker 16?
Does it have steel or damascus barels?
What is the wall thickness at the end of chambers now?
If I had a steel barreled, 2-1/2" chambered Parker 16 with measured minimum wall thicknesses at the end of the chamber of at least 100/1000ths and it had been made after WWI I wouldn't hesitate to shoot modern factory off-the-shelf 2-1/2 DE (Drams Equivalent) 1oz loads through it. Of course if I blow up the gun and lose some fingers it would be my fault.
I wouldn't shoot the 1400 fps 1-1/4oz "Hunting Loads" hawked by the big box gun stores in it.
Finding Remington, Winchester, or Federal in 2-1/2DE 1oz loads is going to be just as much trouble as ordering in from RST though.
Unless you are going to shoot thousands of rounds through the gun at clays there is no need to go into reloading. I would order in a flat of #9s for skeet to learn how to shoot that particular gun and then a flat each of 6s and 8s for hunting.
But back in the seventies I thought light beer was the stupidest idea I had ever heard of and that men would never drink low calorie beer.
