The weird part about the RDA change is that it also depends on the INSIDE of the knuckle being molded in such a way that there's a slight peak in the center (inside). If that's there, it really, really locks in to the opposing RDA 'finger' on the other side and if you pull hard enough, it will break the head off before it will separate.
Certain couplers never had that center mold line/point; I think original low-shank 1104's, likely some others. If there's no center mold line, the RDA fingers probably don't have anything to grip on the other coupler.
You are right. I think that all the couplers with offset (Not centered) shanks are molded with the mold parting line off-center (because of the shank being offset the mold has to be designed differently) So the underslung and the T-shank couplers do not have that vertically-self-centering advantage when coupler to the RDA coupler.
