Much depends on the point at which we decide that RP came into existence. William Gladstone, who was PM for four different terms in the late 19th century, went to Eton College yet his pronunciation gave away that he was from the North of England and his speech was criticised by RP speakers in the early 20th century.
Definition of vane. 1a : a movable device attached to an elevated object (as a spire) for showing the direction of the windb : one that is changeable or inconstant.
On the early 1900s farm, the practical determination of wind conditions and what they could mean, were from first-hand observation, experience and local knowledge. A wind vane (also called a weather vane) was a pointer that freely rotated on the top of a fixed vertical rod. It was designed to swing easily and point to the direction from which the wind was blowing.