The ERA chassis doesn't have the stiffening ribs nor do CAV and RCR.
(The pictures below are taken from the CAV and RCR websites. The first two pics are CAV's chassis, the third RCR's.)
I believe that the current RCR-40 does have the stiffening rib that runs directly across the front of the seats now.. I'm not 100% certain why it was added.. probably to meet some SVA requirement or some such.
Stiff? My chassis is stiffer than anything I've ever seen before - While I can't give you specifics, all I can tell you is that when you jack it up and put it on Jack stands, you always have to shim the jack stands to keep the chassis from rocking.. Not a millimeter of sag or flex in this thing...