Hello Shaun:
Just ran across your post about the truss, and from your description I would say that it is no doubt a bridge truss, and as yu suspect just holds the bottom chord from sagging.
I have run across this type of truss in an 1865 Lutheran church here in Ontario Canada. The truss in the church was fabricated very similar and stood independent of the rafters which towered over the trusses. The difference in the church truss though was that instead of a vertical centre post the centre of the ceiling cross girt was held steady and straight by a vertical 1.8" wrought iron rod. This building has stood for 136 years now and is still nice and straight. The peculiar thing is that the last truss near the front of the church also supports 1\2 the weight of the spire whicch rests on it, but it has been additional strengthened by adding heavier timbers
The Northern Hewer