The original seat in the Sports Renault/Spec Racer/Spec Racer Ford is a fiberglass shell. It doesn't fit anyone particularly well, and long ago an alternative aluminum Butler was approved. The Butler seat is gradually replacing the fiberglass seat, but there are still a lot of the old ones around.
At Lime Rock a couple of weeks ago, and at Pocono this past weekend, I saw a couple of fiberglass seats with cracks in back on the driver's right, down in the well between the seat proper and its mounting to the right side of the car. I don't recall having seen these sorts of cracks before, and they concern me (I wish it had occurred to me to take some pictures; if I see any of these at Lime Rock at the end of September, I'll be sure to take some and post a followup.)
I'm not yet sure how serious this is; I'll probably drop a line to Enterprises to get their take on it. But still, I strongly recommend going with the Butler, it's a much better seat than the old one.