I'm quite surprised by the lack of any technical details or analysis, given that this is PFM.
I smiled as I read through the thread and I'm going to put my money on a miniturised "Bell Nodal Beam" (as found on a 206L Helicopter)
Something setup to cancel the primanry bounce mode of most MC carts, having said that it would follow that different versions for MC and MM carts would be beneficial.
https://www.researchgate.net/figure/7-Nodal-beam-concept-depiction-22_fig3_258544376
just thinking aloud
Edit, Maybe I'm overthinking this and it's nothing more than a device that allows the cart to freely pivot about its roll axis?