Smectics of achiral, tilted bent-core liquid crystal molecules are chiral, but their optical activity is generally small. Here, we study the effect of conformational chirality on optical activity in smectic phases of an achiral, bent core mesogen, W513. The neat material has a modulated B4 phase, which appears dark under crossed polarizers and shows no observable optical rotation under decrossed polarizers. However, mixtures of W513 with a rod-like mesogen, 8CB, show a conventional B4 phase, in which distinct left- and right-handed chiral domains with opposite optical activity are observed. The optical behavior of the mixtures is consistent with NMR results, which show a splitting of the carbonyl peak of the bent-core molecules into two, indicating a twisted conformation between the two molecular arms of the bent-core molecules as in conventional B-4 materials.