If it has a glass plate inside, someone has used the glass plate to hold sheet film flat. There are two ways to do this: The "jam method" - smear the glass with a sticky substance like 3M non-permanent adhesive spray - or orange marmelade, or even blueberry jam in an emergency. The film will stick to this, and the plate/film pack is loaded in the holder just as a glass plate would be.

The other way is to put the film under the glass, which leads to focus difference. check to see if your ground glass is reversed to compensate for this difference - mine was. I now "jam" my film.