John --
It's not the shackles that are different. In fact they are (were) probably original 1959 shackles. One of them needed to be destroyed in the removal process.
The mod I installed is the "Rear Spring Locator" kit -- manufactured by Joe Alexander and available lots of places. Instead of two "soft" bushes in each eye -- originally rubber, now, often replaced with polyurethane -- the kit employs a single bronze bush through the spring eye and the frame eye -- through which the shackle pins are installed.
The bronze bushings were a tight fit in my frame eyes. One of them (left side) needed to be reamed after installation for the shackle pin to even get into it. The other one "seemed" to be OK. That's the one that froze. It is now reamed like the other. They need to be a slightly sloppy fit so you can get lots of grease in there with with the pin.
One more time -- just for clarity. The rubber or poly bushings fail by disintegration. If you get a problem back there, it will come apart as it is supposed to. This bronze bushing failed by becoming too tight on the pin, requiring a lot of extra effort to get it apart for repair. If I wanted to get the bushing out of the frame eye, at this point (body on the frame), I would HAVE TO remove the rear wing and drive it out towards the center of the car, cutting it off as it emerged from the eye, as there is insufficient room for the whole thing to come out of there in one piece. Later for that. I'm going to try it one more time with the increased clearance for the shackle pin. If I have to go through this again, though, then out they will come!