What is a 710 knob?

A few days ago I was having some work done at my local garage. A blonde came in and asked for a seven-hundred-ten knob.

We all looked at each other and another customer asked, 'What is a seven-hundred-ten knob?' She replied, 'You know, the little piece in the middle of the engine, I have lost it and need a new one..' She replied that she did not know exactly what it was, but this piece had always been there.

The mechanic gave her a piece of paper and a pen and asked her to draw what the piece looked like. She drew a circle and in the middle of it wrote 710. He then took her over to another car which had its hood up and asked 'is there a 710 on this car?' She pointed and said, 'Of course, its right there.' If you're not sure what a 710 is...