What WEDJ said, plus the anhedral stab in knife edge counteracts the tendency to roll out of knife edge. This tendency is caused by 1. the wing dihedral and 2. most of the fuselage side area being above the centre of mass.

Also, there did exist a Curare without an anhedral stab. The Prettner designs previous to the Curare were the Sicroly series. The last of those, the Super Sicroly 2, was pretty much a Curare without the anhedral stab, and the stab placed lower on the fuselage. Hanno Prettner won the '75 TOC with the the Super Sicroly 2, before winning 2nd place in the '75 World Champs, 1st place in the '76 TOC and 1st place in the '77 World Champs with the Curare.

G.