Hi, just been reading this and thought I'd offer my findings though I'm not sure if it helps at all.
In my time I've owned and worked on several Senators and Carltons as well as this Omega and they all had the same rear calipers with solid discs fitted though no doubt GM gave them different part numbers.
A work mate asked me to look at his('98 omega) brakes as they were squealing, they just need a clean and free up. The rattle springs and pins were past their best though so I offered to fit some good second hand ones I had in the garage, only I couldn't because they were too short.
Now, these pins came off of my Omega when I replaced the pads and I wanted shiny new ones in there cos I'm silly like that. I had to return the first lot of new pins however because they were too long, and when the replacements arrived the packaging said for vx carlton '89 onwards.
All this leads me to believe that GM fitted whatever was in the parts bin at the time which in turn leads to the situation above.
There are two different calipers for solid discs and one for vented discs even though officially only two were fitted and at some point in the past your car has had one replaced.
One last thought, are carlton/senator pads thinner?
Either way, I hope you get it sorted.
Simon.