Hi Simon;
Welcome to the wild world of historic carpentry!
The unusual "horizontal corner braces" are sometimes called angle ties or if associated with a dragon beam a dragon tie. Historically a corner was called an angle so if you see a historical reference to an "angle rafter" it is referring to a hip rafter. I associate angle ties with very old American buildings so it is very interesting to see one in a modern-enough building to have circular sawn timbers.
This frame is also a level assembly frame. That is the plate and tie beam are at the same (or similar) elevation and the tie often tenons into the top plate. The tie and plate being at the same elevation allow the angle tie to be easily framed in.
The purpose of an angle tie is to prevent racking of the plate and tie beam, a condition which is not widely documented but seems rare.
Sugar houses have the unique feature of roof vents which often form the shape of a monitor roof. It is interesting to see how this one was built.
Thanks for sharing these interesting photos.
Jim
