Before revision:  I show that in both cases the coefficient on treatment is identical to a convex combination of two other parameters(different for OLS and 2SLS), which can be interpreted as the average treatment effects on the treated and controls under additional assumptions.
After revision:  I show that the treatment coefficient is a convex combination of two other parameters(different for OLS and 2SLS), which can be interpreted as the average treatment effects on the treated and controls under additional assumptions.