this mysterious WNM corridor has practically been sitting in front of our eyes for the past decades (well, i.e. for everybody who owns the Whitfield Making of Star Trek).
On page 11 of the first picture section in the book, the top photo shows the end of the corridor (not steps but a ramp!), the bottom one the angled-in corridor at the other end. If someone has taken his Making of apart, maybe he (or she) could be so kind to scan these b&w pictures in their full detail glory.