What is a 710?

What is a 710?
A few days ago a friend was having some work done at our local garage. A blonde came in and asked for a seven-hundred-ten.
Everyone looked at each other and another customer asked, 'What is a seven-hundred-ten?'
She replied, 'You know, the little piece in the middle of the engine, I have lost it and need a new one...' She said 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 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, it's right there.'
If you're not sure what a 710 is... go to www.hotautoweb.com/cogifs/710.jpg