Hi Guys
Turns out what I purchased seems to be identical to the original. I didn't aim to do that, I just searched for reversing alarms. Found one at 107db, and bought it. Hole centres were the same, so, result!
Not clear why the old one would be so quiet now, but I made a video to compare. Not a direct comparison, since the new one was already mounted when I made the recording, and the video doesn't do a perfect job of reproducing the sounds, but I hear the new one well, very well, when fitted inside the mower, and can barely hear the old one, right in front of the camera, even though I had cleaned out the majority of the muck over the sounder. It would seem my hearing isn't as bad as I thought, lol. The new one is very loud, in real life, and can easily be heard over the sound of the engine. Unless your PC has good sound, you might not hear the original.
https://www.youtube.com/watch?v=56Jxl7-xSks
Happy with the outcome, and thanks for the search suggestion Alvy, which pointed me at the right product.