Looks like New England Power Company - the highlighted parcel, as well as the W chunk on the N edge and the parcels to the S.Groton 4 Corners area
Since I've started mapping more, I've discovered town/city tax maps to research lot boundaries and ownership.
Also found Acre Value - no-cost basic sign on, shows a lot of parcel information, with some gaps depending on their sources. Such as crossing from Groton into Ayer, there's some blanks. Linked view is from there.
Just found MA Interactive Property Map
on arcgis. Zoom in to the area of interest, and click on a parcel.