Robert,
The module you describe is the same type I have here - three of them in total. It's essentially a high-pass filter which goes into the Aux Band filter spots on the RF Unit. This board along with a few diodes allows the Gen VFO to transmit and uses the highest-frequency low-pass filter chain in the rig to reduce spurious emissions.
It's a kludge at best. Might work for the transverter/CB crowd that wants output from 24-30MHz but on 60M I'd be leery of the 3rd and 5th-order harmonics.
The PROM I mentioned goes on the CPU Unit in a socket. You also need to install diodes on the RF Unit along with discrete filter components (i.e., capacitors and tunable inductors) and you end up with 3 extra 500KHz segments. These segments are documented in the FT-980 CAT reference, so provision for addressing them via the CPU is present in the firmware.
73 - Fred, N8YX
