The aperature is missing and these came in different diameters and openings. It's obvious that your rifle was made by Mossberg as a "Store-Brand". I have seen Marble's and Mossbergs on these rifles or at least, that is what I have on mine. Can't tell what the make of yours might be. Go to GB or Ebay and look for
aperatures. Let me know how else I can help. ...
Be Safe !!!