I found the following maps on my CD-ROM:

http://www.realmbuilder.com/tristate/ProvinceIsland.html shows the
island in Lake Memphemagog that Mike mentioned. The Canada/US
boundary across it is about 80 meters long and is marked by a
monument. The island is named Province Island.

http://www.realmbuilder.com/tristate/ProvincePoint.html shows a
peninsula in Lake Champlain. The Canada/US boundary across it is
about 100 meters long and is marked by a monument. The peninsula is
named Province Point, not to be confused with Province Island.

BJB