None, they represent the same unit. Original King Radio part numbers were 9 digits. For example, 066-3056-01. During the Bendix and King merger (i.e. Bendix/King), a new part numbering system was created that converted these 9 digit part numbers to 12 digits. Therefore, 066-3056-01 became 066-03056-0001. Despite this numbering change, units that were originally from the King Radio design still have the 9 digit part number format on the unit dataplate. The 12 digit format for King units appears to be used for catalog and internal Honeywell purposes only. Therefore, any unit that has a zero in its third to last number (i.e. XXX-XXXX-X0XX) has a 9 digit part number (i.e. XXX-XXXX-XXX) on its dataplate / ID Plate.

In January 1989 Bendix/King changed from a nine digit to a twelve digit part numbering system. The new, larger 12 digit numbers allowed for the inclusion of software version into the last two digits of the part number for certain units in which software changed frequently such as EFIS and TCAS processors. Therefore, the two digits immediately preceding the software version indicate the hardware version of the unit.

Different software versions imply different operational features and/or interface capabilities and software modifications imply software repairs (bug fixes) to insure proper operation of these features and interfaces. Software version upgrades frequently require hardware modifications to the unit. Such hardware modifications accompanying software version upgrades do not necessarily change the hardware version of the unit.

Not completely. The -01 versions include a stepper motor that is driven from the directional gyro within the compass system. Therefore, when the aircraft turns, both the compass card on the HSI and ADF indicator will turn accordingly. Unlike an RMI, however, these ADF indicators must be synchronized to the HSI compass card by rotating the SYNC knob until its heading matches the HSI. An RMI automatically updates the heading on the compass card.