For use with System 55X. P/N 01188 for non-FD systems (optional), P/N 01188-1 required when interfacing ST-361 S-TEC FD, P/N 01188-2 required when interfacing to an Aspen or King KI-256 FD, P/N 01188-3 required when interfacing to Collins 329B-7R FD.