Hi Will n Tim,
The thresholds that I have seen are simply nailed on to the bottom of the posts - two per each side to create a slot into which the threshold planks are lowered onto the threshing floor. I think that Tim is correct in that winnowing is the process of lifting the threshed straw with a fork to separate the wheat or other grains from the chaff. A hinged stick was used to thrash the corn called a "flail" as in "flailed alive". The separated grain would have then been swept up, seived, measured, bagged and stored in the granary. Winnowing barns generally have their long walls faced into the prevailing wind to encourage a nice draft to blow across the threshing floor, exiting the barn via a door on the opposite side, the draft carrying with it any dust and small chaff. This must have been a very tedious process and one which was oft times monitored by a tallyman to prevent theft of the grain.
Regards
Ken Hume