Your memory has not played tricks on you, there is a Koppel dump car (and several Koppel chassises) at Sheepscot.
Just reading this great information about the industrial railway at the American Thread Co. in Milo. After many years if not having an internet connection and working nights, I constantly find something new in this discussion forum everyday. I equate that thrill to (say) being given a big stack of old 1950's (ish) railroad magazines.
Anyway...regarding the Koppel tip cars and chassis at the W.W. & F. Ryan. Museum, one complete car with dump body/hopper came from the Ramsdell estate in W. Thompson, CT. and therefore I would assume it to once have done duty on the original W.W. & F. Ryan. Recall that Linwood Moody is shown in a photo at the Wiscasset shop, with such a car among the abandoned equipment.
Two Koppel tip car chassis were brought up to Sheepscot (with the Brookville) from Paine Enterprises Marine Ry. that was a 1,000 foot operation in South Wellfleet, MA. The Brookville and chassis were used to convey beach-goers, coolers and picnic baskets from a well made shop building, right down to waters edge.
The wooden tip car decks were rotted out when a group of museum members salvaged the rail and equipment. Later, steel plate was applied temporarily and stake body sides attached for use hauling small batches if crushed stone, etc.
The steel plate decking was later removed and the chassis were then used as shop trucks, for the caboose.
There was a 4 ton Plymouth gas/mech'l engine in the So. Wellfleet shop, and I heard it run after doing an oil change on it. The wheel flanges on one side were worn to a knifes edge however. Mr. Paine told me he sold that engine to the Beaver Brook Museum, though it may have actually gone elsewhere.