Interesting. Seems impossible. the droops effectivly change the tip airfoil from symm to a very high lift undercambered section.

Of cource they are made to blend into the semy-symm airfoil section on the P-51 PTS so who knows what the section is on the tip of the ST50. I'd thing the benefit your seeing is more due to the added area at the tip than a change in Af section.