Obviously all I did was copy and paste from your own link LOL. Here’s some more from it for you to trip out on.
“The plate was handed to Washington, who stepped down into the foundation trench, laid the plate on the ground, and lowered the cornerstone onto it.”
The cornerstone and silver plate are two different things. Anyway, it’s irrelevant, George Washington was a Mason as were many of the Founding Fathers. He laid the cornerstone that’s a historic fact. The plate was probably stolen by workmen (or Congressman!), or the British when they burned Washington or by any number of other workmen from the numerous times the Capitol has been repaired and renovated.