Let Q be a lower finite
quasi-ordered set and let I(Q) be the incidence algebra of Q over a field K. In this
paper we determine all faithful distributive modules over I(Q) and relate the result
to the structure of the outer automorphism group of the algebra. In the case when Q
is finite we also determine all left ideals L of I(Q) such that I(Q)I(Q)∕L is a faithful
distributive module over I(Q).