Hi Roger, so you're talking about these edges here:



If you zoom in a ways on those, you'll be able to see that they're actually part of a thin little slivery surface:



That's actually a separate object from your main other piece, so just select it and delete it to get rid of it, I've attached a version of your model with that deleted.


Usually when you see some kind of mysterious extra edges where you can't see the surface, it means you've got some kind of thin little slivery surface there, which are hard to see unless you zoom in closely to them.

- Michael