These are all just replicas. To my knowledge, only one or two of the original mile markers survived, and this is not it.
The surviving masonry is visible in the photo above (foreground). Beyond it is the viaduct, which was blasted away leaving only traces of its existence behind...
I've circled the surviving arches in this photo...
Mitchell Point Tunnel & VIaduct, 1920s, from water... Scott Cook, Curious Gorge Blog. http://curiousgorgeblog.wordpress.com/36-mitchell-point/