Well, I'm feeling rather stupid at the moment. The piece in question has nothing to do with the lawnmower. We recently bought a couple of carving knives (Thanksgiving coming and all). The piece just happened to fall out of a garbage bag within 2 feet of where I was working on the lawn mower in...
After replacing a broken spark plug and changing the air filter I found the pictured piece of plastic on the ground. Any idea of what it is and where it goes?