There was no existing HDD. It was empty, gonzo, nada, null and void, non existent. It's up and running with the new SSHD as described, but this new drive seems to be too small as it there is space under it and it hangs down (or up, depending on your point of view) with nothing holding the unit into the slots. Putting the back cover on will not prevent it from coming loose inside.
I'm surmising it needs some kind of spacer or something?
Hello again. Finally getting around to finishing this....
There was no brackets or screws. Take the cover off and it was an empty void where the original drive was.
Those spacer adapters you linked above solve half the problem. While they would space the drive unit up so it sits nice and square with its plug in, this one still needs something at the back to hold it in place from working out of the plug. There is about a 1/4" gap between the end of the drive and the inside face of the drive bay.