It might have something to do with the way the head is supported internally. The older ones had a support bracket that used to span across the chopper's mouth and connect to the "chopper tube" from the Thunderbolt. The tube itself would sit right in front of the intake. This in combination of the shorter back probably introduced some turbulence issues and inhibited airflow which is probably what bottle-necked the third generation SRN and SRNB to 128 dB. When they made the back longer they shortened the bracket and placed the chopper higher up so the tube wouldn't be in the way - might be the reason 130s and Equinoxes have a hard time holding their pitch in high winds. The longer back probably puts a little more weight onto the head causing them to have a slight lean. Other than that they are pretty much the same. I haven't noticed much of any lean on the older ones around here, but I never payed that much detail to the heads. The one above probably had it's support bracket bracket break from the shock of the impact.
Inside of the SRN. All of them from the "Roundback" to the SRNB were built this way. Credit to Jeb M.
The inside of the 130. I suspect the Equinox looks identical to this, just with an Eclipse rotor in place of the Model 3's rotor. Credit to Christopher J.
