[plt-scheme] Inspectors in mzscheme

The following code does not behaves the same under drscheme and mzscheme
with respect to inspectors:

   (define-struct loc (x y) (make-inspector))
   (display (format "~a~n" (make-loc 4 5)))

Under mzscheme, it prints the opaque value:
    #<struct:loc>

Under drscheme, it prints the open value:
    #(struct:loc 4 5)

I'm not sure I understand why there is a difference.
Is it possible to get the open value behavior under mzscheme?

-- 
"The thing I remember most about America is that it's silly.
 That can be quite a relief at times."  -- Thom Yorke, Radiohead

- Guillaume