David,
I agree with your conclusion about there being no Q&TL connection to this car.
The person who saved and/or created this coach did also acquire some equipment from the Quincy & Torch Lake RR, but there is nothing about this coach that I can identify as possibly coming from anything owned by the Q&TL.
Lacking any clear explanation of this car’s origin, I am considering some evidence that this coach is an original car from the Duluth South Shore & Atlantic RR. It may be that the entire car today came from that origin completely intact, and was therefore not built in this modern era as a replica.
However, the plausibility of the DSS&A ownership of the car needs further exploration because DSS&A was built as standard gage. Generally, the DSS&A linkage points to an origin of the car at the Mineral Range RR or the Hancock & Calumet RR, both of which were narrow gage.
If this origin theory proves correct, the most provocative mystery will be how an 1880 car that was displaced from its original owners around 1900 happened to survive intact for another 60-70 years.