<< but I'm not convinced that an additional 17mm or so of rubber in the lengthwise direction is a solution.>>
V.,
Except for leaky rear pinion seal damaged ones , I have never seen a solid core one fail , but I have seen all the original 2 piece fail, leaky seal or not.
The solid core ones may also use a better bonding process, but I do not know that for sure , I only can tell you that they have seemed to have rectified the known problem of the 2 piece with the solid core design.
Not to say they won't fail.... just posting my experience/observations w/swing axles .
It would be interesting to see the results of your coupling calculations and test.
Also of interest would be the actual difference with the 110 350 13 75. You have an early .043 and according to 250sl Chassis Edition "A" , [ Early 250 specific],this part was an Option . I would think that means it would be part of the package when requesting the heavier suspension . There is probably someone here that would know
more on that than I do. Might even want to mention that part# to the outfit you got the 12 75 from. They may know the exact difference.
I am only familiar with aftermarket solid case .
These are also available here at K&K, so you may still have time to get one overnight, if need be.
Jim.
The 121 chassis is a different part. The front sleeve uses a seperate design.