Holy cow looks like you hit the jackpot. They did something similar to an old mine pit here in MN a couple years ago with rainbows and browns, in the 17-20in range. I think they were older fish that they used to gather eggs and milt.
Trout hatcheries do this quite a bit. They keep the brood fish so long and then stock them in ponds/lakes where there is a good chance they will get caught. Next week there will be a pond near where I live stocked with brood cull brook and golden trout.