Is it this ? Framelock Clipset.
https://mountainbuggy.com/uk/Buy/parts/parts-for-legacy-models/duo-parts
Edit
Or maybe this? https://mountainbuggy.com/uk/Buy/parts/parts-for-legacy-models/duo-parts/harness-buckle
Are you just randomly picking items from that site that look nothing like the picture in the OP?![]()
No wait, it's yellow and black, and it's in GD!![]()