For all those hardcode node name, I would recommend you inspect the exported onnx model in netron.app, you can read the node name and input/output there.

The concat_291 is the node right before the original export result. While the onnx::concat_458 and 459 is after running the stitching nms, you likely should see the dangling output like in the diagram

Also, I was working on the nano model, I believe if you use larger model, all those node names would be different