In 1922, The Metropolitan Museum produced a film to promote its arms and armor collection, using various parts of the northern part of Central Park, including the receiving reservoir that is now the Great Lawn.
NYC Water's Flickr site posted two film stills from the video, seen here.
We believe the first half of the video shows the Receiving Reservoir effluent chamber access adjacent to the Metropolitan Museum. It would be approx E.80 or 81St. where water left the reservoir in pipes if there was a gated chamber there. Alternately, it could be the reservoir influent chamber access on W. 85th St.
You can jump through the film whose link is attached to the caption on NYC Water. The film also uses Belvedere Castle as a stand in for medieval scenery, showing an armored knight riding a horse along the southern border of the reservoir.
We are still chuckling over the fact that we have a film of the old reservoir used in such an imaginative way.
View the film HERE