Hi Mark, so there are a few different little slivery surfaces in that area shown above, it's kind of a mess in that spot. But the area on the opposite side is completely clean. Since it looks like this piece is supposed to be symmetrical probably the easiest way to finish it would be to cut it in half with the Trim command and mirror the good half and then join those pieces together, all done after deleting the faces that you don't want for the inner portion.

.... But also looking at it a bit more the piece seems to be slightly rotated, like the top edge is not completely flat going along the x axis, it's like 1 degree rotated from horizontal. So that could potentially complicate making a good mirror of it.

- Michael