there are a couple of places:

https://bostonmaine.squarespace.com/mecvalplans Maine Central Valuation Plans

https://digitalmaine.com/archives/ Maine State Archives

https://www.loc.gov/maps/collections/ Library of Congress

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Another place to look is to search for Sanborn Maps of the area you are looking for - most were done between 1880s and 1930s, but only "urban" areas, like Dover-Foxcroft or Skowhegan or such.

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You might also check the Penobscot County Registry for the valuation ("val") plans on that stretch - I know they're in there, but it might take a bit of digging.

https://penobscotdeeds.com/ALIS/WW400R.HTM?WSIQTP=SY00

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ETA: you might also check with a local survey firm, or the Maine Society of Land Surveyors.