I'm not exactly sure and maybe one of the Rares guys will correct me if I'm wrong, but I think the difference is at the top of the metal spline that's embedded in the rubber.
On a Monaro, there's a screw that locates the top of the rubber to the chrome metal pillar that you spoke of. Sedans don't have that extra screw. The pic below shows the pop rivet that holds the top of the rubber to the frame and the hole at the top is where that screw goes.
I've done a few Monaro quarter rubbers over the years, so might put my two bob's worth in on how to do it or, more importantly, what traps there are in doing them
First you have to determine where the hinge support (the stainless bit that's riveted onto the frame) goes through the rubber and slit the rubber with a Stanley knife. They aren't slit in production.
Then, make sure the holes are in place where the pivot goes through the rubber and where the locating screw behind the pivot goes. Sometimes a thin layer of rubber covers these holes.
Next, apply a liberal coating of rubber grease to the outside of the new rubber, where it locates in the channels of the frame and insert the rubber. You'll probably find that you have to manoeuvre the rubber into its exact place, the rubber grease helps.
Before bending over the locating tabs in the metal spline, check to see if the top rivet hole lines up with the hole in the frame. I've usually had to file or drill out this hole because it doesn't line up.
I think the rest of the process you'll figure out while you're doing it.
I've tried a few different ways to swage over the hinge rivet. I've settled on doing it in a vice.
Place a ball bearing about 4mm diameter in the rivet opening and start doing up the vice. As the rivet starts to "belly out" remove the ball bearing and just use the vice jaws to squeeze it the rest of the way. You might need about 3 hands to hold everything in place while you're doing this.
