Well, every day is a school day, as I didn't know that a Nav 5/6 would not fit into the old 4-button mount. If the 5th digit of the part number printed on the back is an 8 (e.g. 77528) then it is a Nav 5 or 6. Since the Nav 5 was contemporary with the introduction of the Multi-controller - aka Wonderwheel - the 4-button mount would have become obsolete.
To the OP, good luck with Wapping's suggestions.