  Differential modules over a commutative differential ring R which are
finitely generated projective as ring modules, with differential homomorphisms,
form an additive category, so their isomorphism classes form a monoid. We study
the quotient monoid of this monoid by the submonoid of isomorphism classes of
free modules with component wise derivation. This quotient monoid has the
reduced K group of R (ignoring the derivation) as an image and contains the
reduced K group of the constants of R as its subgroup of units. This monoid
provides a description of the isomorphism classes of differential projective R
modules up to an equivalence.
