In the American regionalism
Put 'er there, pardner! (i.e. Let's shake hands)
why 'er ?
P.S. When someone is manipulating equipment, such as a crane or a hoist, to move a heavy load, someone directing operations might yell the command "Put 'er down! Put 'er down!" if he notices that the load has become imbalanced. Or he might be motioning the crane operator to keep moving in a certain direction, and then say, "OK. Put 'er there."
Are these two uses related?