From the meager info I've been able to find, the PCII was a less sophisticated unit than the PCIII series.
The PCIIIr has the same injector timing connectors as the PCIIIUSB, but also has an extra connector that interconnects with the ignition pulse generator since it had the capability to alter ignition timing.
The PCIIIUSB doesn't have that connector, only the ones that interconnect with the ECU and injector harness.
The PCIIIr is also smaller than the PCIIIUSB, probably almost the size of the PCV.
I have no idea why the PCIIIUSB is so much larger (approx. 30%) than the IIIr or V.