Hi wisty,

re:
> Thanks. That did it. Still not sure why an (apparently) identical hole on the opposite side deleted without trouble.

It's because the hole on the other side did not go through the "seam edge" of the closed surface:


Currently to do an "untrim" and delete a trim boundary you have to select all the edges belonging to the entire boundary.

To do it on this side you'll need to select the face and use Edit > Separate to break it out from the main object. Then select one edge of the hole you want to delete, then press Ctrl+A to do a "select all" which will select all edges of that surface and now you can use Delete to remove that boundary and recover the full underlying surface.

Then select the pieces and use Edit > Join to glue them back into a solid again.

I do want to improve the "untrim" function so you can remove part of a trimming boundary instead of the whole boundary but currently it only works on the whole boundary.

- Michael