The US2 supports any Humminbird unit capable of using the 200/83kHz DualBeam sonar. Here are the current Humminbird sonar-GPS models that can use the US2:
386ci Combo
597ci HD Combo
788ci HD Combo
858c HD
958c HD 1158c

There are many more older model Humminbird units that can also use the 200/83kHz DualBeam sonar and have GPS like the 581i Combo you mentioned.