You sound more local than me!
As I hinted at not clearly, the fedgov does not use punctuation and the Board of Geonames just removes them from historic names.   So add to the list the GNIS: https://edits.nationalmap.gov/apps/gaz-domestic/public/search/names/611869
would make sense to have an alternative name of Sandy Pond since GNIS lists it.   as for the dam, if the dam is actually called sandy pond dam by the manager, the town, and the MassGIS data, then it makes sense to use that name and make flints pond dam an altname IF there is local culture of calling it that.   if the world is inconsistent in naming, that's how it is, and we should just capture that.   If you know the conservation department people asking them might be interesting.   I'll try to dig some more tomorrow.

Not trying to give you a hard time - it's great to have people digging in and trying to get these things really right.  It's what makes OSM data so high quality.