Ok, here's the real scoop.

Prior to Spring 2002, RealFlight G2 was sold with either parallel port interface or a game port controller. The software was protected from copying using one of the strong protection schemes. Once this protection was broken, some indiscrete people started copying RealFlight G2 CD-s and using the sim with 3rd-party controllers or even regular joysticks.

The response of KnifeEdge, the developer of this sim, was to add a special USB controller and rely on it as on a copy-protection-device: the sim will not start without this device attached to the PC. They went to great length to make this new Interlink controller hard to imitate, so that it's technically difficult to make another USB device, which will present itself as an Interlink controller. Fortunately, Interlink controller is quite a decent piece of hardware, it's worth every penny and AFAIK it comes with G2-Interlink upgrade disk.

The difference between pre-Interlink G2 and G2 Interlink is rather small. All G2 upgrades can be applied to pre-Interlink G2 to bring it up to date. All Add-Ons work with pre-Interlink G2 too.