My understanding is that Ron's hand-tuned final calibration is unique to each instrument. The mass and location of your brass hardware, the specific windings and location of the pickups, etc, all have an effect on his final hum-cancelling tweak. So there is a chance that swapping cards and/or wiring harnesses between your S-I and S-II could introduce some noise issues.
That said, apart from that last tweak the components should be interchangeable. A quick experiment would be to move the preamp card from one to another and see what happens. Any increase in noise and hum?
Let us know what you discover.
Jimmy j