Photo provided by Keene Library & Historical Society of Chesire County
View this photo on Flickr
BH Photo #269817
Looks like the pin migrated north of where it should be. This bridge was located adjacent to the stone arch railroad bridge, and was replaced in 1930 by the Vilas Bridge. Thus it belongs in both Chesire County NH and Windham County, VT.
Here's a picture I found showing its location, you can see the arch rings of the railroad bridge in the backround.