  New expressions for the multipole moments of an isolated post-Newtonian
source, in the form of surface integrals in the outer near-zone, are derived.
As an application we compute the ``source'' quadrupole moment of a
Schwarzschild solution boosted to uniform velocity, at the third post-Newtonian
(3PN) order. We show that the consideration of this boosted Schwarzschild
solution (BSS) is enough to uniquely determine one of the ambiguity parameters
in the recent computation of the gravitational wave generation by compact
binaries at 3PN order: zeta=-7/33. We argue that this value is the only one for
which the Poincar\'e invariance of the 3PN wave generation formalism is
realized. As a check, we confirm the value of zeta by a different method, based
on the far-zone expansion of the BSS at fixed retarded time, and a calculation
of the relevant non-linear multipole interactions in the external metric at the
3PN order.
