There were several boundary disputes over different areas of the Landing. This boundary issue started in the early 1800s and lasted nearly until 1900. It also caused some issues with overlapping responsibilities of the census takers, which is why you may see two different addresses in the census recoeds of that era.
The Union Church was at the top of Weymouth Landing, and the boundary line between Weymouth and Braintree used to run directly through the church. This church kept many records of Weymouth Landing residents. The Union Church burned down in 1897 and was rebuilt a little further up which means it is now completely in Braintree.
Weymouth and Braintree were originally part of Suffolk County. Both towns later joined Norfolk County.