@dkwolf, thanks for your message. I'll reply here just to keep communication in one place:

> I do not have any recollection of that boundary. Please edit if you see fit. While addressing boundaries of nature preserves, I used both the usgs and granit and town gis to make any boundary changes.

I've done some investigating by consulting local expert on perambulation Kurt Gotthardt. The cause is quite fascinating and we were both right in a way.

The stone monument is the true 4 corners. As an artificial monument it controls over the original paper description of length and bearings. However, the 1927 USGS topo map used an approximate location for the corner and that error has propagated since to be in OSM, GRANIT, Google Maps, and even local town GIS systems. On USGS maps through 1996, there are markers for monuments. You will notice this corner lacks a monument and is therefore approximate.

I will fix this specific instance and also likely write a blog post about this surprising finding.

It appears that such errors might be widespread, and it would be interesting to consider ways to know whether a boundary node is approximate or monumented in OSM.

Who knew no one knew!