[Tom Goldman]

The rule book is vague in regards to stock seats ,it only directly mentions aftermarket aluminum or fiberglass seats requiring attachment to the crossbar, it does not specifically mention factory seats. See General Regulations section 6.2
Even the new COPO's come without the seat attached to the crossbar.
That being said , I have always attached the factory seats to the cross bar.
It is a simple thing to do and increases your personal safety.