The big difference is that the Modem IB lacks the unusual ability of the original Modem I to be used through the TRS-80 Model I cassette interface (with no expansion interface nor RS-232 interface of any kind).
Haven't seen a service manual, but it should be less mysterious than the Modem I, as it's just a standard RS-232 <-> Bell 103 device, not doing anything Tandy-specific.