Aha! I think Gorbas has the answer

It all makes sense now.... So, the focussing ground-glass (not film-flattening anti-Newton) is removable and then would be replaced by a dark-slide holding the film.

Making an exposure would require a shutter of some sort, but that could be as simple as removing the lens cap and counting elephants (though how that light-level would be bright enough to focus I'm not sure) or by using flash for the exposure and modelling light for focussing. When I used a copy-camera regularly, also decades ago, the lens was mounted in a shutter and the flash was synched in the usual way.