After playing around with the output stage I realised the transducer is connected to the IC with no marking. As was the output from the micro-controller. I'm sure it was only fair that I made the assumption that the unknown IC has some form out output stage to drive the transducer...

WELL, NOPE!

The IC was a straight pass through. No semiconductor between input and output. So a dual MOSFET driver to the rescue.

The ultrasonic sensor was mounted on a wooden jig to keep a consistent reflection.