Got it! Made a whole new edge and deleted the old ones.

I immediately see the problem, I forgot to delete the inside faces. Now that I've got duplicate edges and vertices on top of each other, any idea on how to delete them without history?

I feel...

So I've modelled one of 7 parts of a gun intended to be animated for character use, pretty simple design without any ngons (so far as I can tell.)

I aligned most of the vertices to similar heights...

Nothing to see here, move along.

Fixed it myself, doubt anyone else would have the same problem.