You may be thinking of the Welcome Mathewson rifle that still belongs to his direct decedent who actually lives across the street from the site of Mathewson's home and shop. They also have his decorated powder horn, his day book and his fowler (which, interestingly, is a Philip Bond, London gun). I forget what Bill appraised it at but, at the time, it was at least 5 times what any NE rifle had ever brought and set off a frenzy of speculation and raised prices.
I've seen and handled the rifle. It is very nice and I am sure it is worth a premium given the association with the maker but I have my reservations about how many people are prepared to pay mid-5 figures for any NE rifle. Bill was extremely good at selling nice guns to wealthy non-gun collectors, folks who were accustomed to paying big prices for furniture, paintings, silver etc and to whom his prices seemed almost inconsequential.