What is the significance of the boot marks, the card and the wedding ring in scene 7 and 8 act 1

The wedding ring is a major clue because it was taken off and removed.... and yet, the other ring was returned to his hand.

“Then the murderer, or whoever it was, first took off this ring you call the nugget ring, then the wedding ring, and afterwards put the nugget ring back again.”

In addition to the other items, Holmes believes that the entire affair is possibly a set up.

“The open window, the blood on the sill, the queer card, the marks of boots in the corner, the gun!”

“Nothing there that could not have been arranged. Mr. Douglas was an American, or had lived long in America. So had Mr. Barker. You don’t need to import an American from outside in order to account for American doings.”