October 5, 1999
I digitized the photo below, taken the day the arch was topped-off (you can see the final section being moved into place) and fitted the measurements to a catenary getting:
The red circles are the measurements (I digitized the outside of the arch), the red curve is the fit and the yellow curve is the published catenary.
The photo is a scan of the back cover of the booklet: "The Building of the Arch" by Robert Artega which is published by the Jefferson National Expansion Historical Association.
Here's a chain (upside-down picture) which hangs 20-in below a line connecting the two suspension points, also separated by 20 inches. The pix was digitized and fit to a catenary giving: