This is an inside view of a boat hull. Paul Stubing indicated that this was the shaft log and engine bed of CR & M. There is a sledge hammer resting upright in the top center of the picture.
Ballard, W.H., “Shaft Log and Engine Bed of CR & M,” Mount Desert Island Historical Society, accessed November 21, 2024, https://mdihs.digitalarchive.us/items/show/6751.Item 7117