I took another look at my Duopar. It has the fancy split bolt with a wedge screw just like the Ti model, though it's an Eco. The threads of the main bolt are stripped. I'm thinking an expander wedge on the bolt is an interesting and clever solution to mounting a RD while allowing it to swing, but ultimately a bad one.

I don't even remember where this RD came from. It's been in my bike parts bin for 25 years. I only rarely came across Duopars BITD, and I don't recall ever installing one either.