The RA1792
Page 11
Optional External Interfaces

As far as I can ascertain, the boards pictured below are the only option boards for directly interfacing with the Processor Board.

Industry standard
IEEE-488 (GPIB) Interface
Software Control Of Racal Equipment

This card allows the receiver to be controlled remotely by the MA1075 Controller. It also allows two receivers to be linked for space diversity operation.
Channel Interface Board

This card provides for extended control of 100-channel selection with revertive indication and band-search or clarifier operation in conjunction with units such as the MA1100 Operator Terminal Unit or the MA1107 Automatic Search Unit.