Yep, they are trivets. All of them but one appear to be ones used to put on a table upon which to place a pot. The smallest one might be one to put INSIDE a large pot or Dutch oven to keep a cake or bread pan away from the sides and bottom so it will be less likely to burn. Neat find, @Ken Anderson
I have a bread/cake insert for my crock pot: Online pics show that it came with a rack to keep it off of the bottom of the crock pot, but I use a trivet.