I've seen mortises near the bottom of the main posts in the barn at Great Coxwell.
We've used a similar mortise when a tall object (in this case our gin pole) was laying down that we wanted to stand upright. If you insert a smaller post in the mortise near the foot of the post, then run the rope up and over the smaller post and down to the people pulling on the rope, then you can get a vertical lifting force from pulling a rope to aid those who are lifting manually. as the pole or bent comes up, eventually the smaller lifting post moves downward until it is no longer guiding the rope and the rope is pulling straight from the top of the object to the pullers.
I personally think that is what those mortises are for, but in the absence of proof...
