The picture certainly looks like the wrong device has been put into the mount.
From memory, the Navigator IV and similar, had a flat back panel. The device in the picture seems to have an angled back panel.
Me?
A. Try squeezing the top the device towards the front of the mount, taking pressure off the release mechanism, whilst pushing the release lever.
B. Remove the mount and the fouled device, as working on the release might well be easier than with it affixed to the bike.
C. Try gently levering something between the device and the mount to GENTLY try to force the device out, whilst pressing the release lever.