Removing an Airmar transducer

[tce001]

I’ve got a rather odd Airmar depth transducer on my boat. From the picture below, you can see that it has no locking nut for the top of the transducer.
So there is nothing to grab onto to remove it. There is a small notch on the forward edge of the housing, but pressing into the notch with a small screwdriver had no effect.

Does anyone know how to remove this type of transducer? And do you also know if a new DST-800 triducer will fit into the housing?

Thanks!