Its spec rule for sure as you knowe..like the "Funny car" cage is 25.1 certification...
you will not be able to pull up any SFI spec book on-line because of the trademark rights..there are several guys on here that could answer the question more in depth..Ed ( mcracecars) could help you answer any questions on this issue i am sure.
Brian
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Bjuice..
"I'M YOUR HUCKLEBERRY ! "