Here's what Carolyn had to say:
This was inspired by some matching activities that other moms set up for their kids on a Montessori Facebook group I'm in.
I gathered up a handful of toys and traced them on a sheet of computer paper, then went over the lines in permanent ink so they would be easily visible. I called Wyler in to check it out, and showed him how all they toys had a matching outline. He caught on pretty quickly and had everything nicely matched. He pulled everything off and did it again. Only his hammer got turned around. It was backwards according to the outline.
He started to rotate it in different directions, but didn't realize he needed to flip it. After a couple tries I flipped it for him, though he probably would have figured it out pretty quick.
I have it set up on a tray (cookie sheet) on a book shelf so he can go back to it when he wants.