  This paper completes the work, initiated in [hep-th/9906003,hep-th/0301204],
further referred as Parts I and II, concerning to Dirac's quantization of
Nambu-Goto theory of open string, formulated in the space-time of dimension
d=4. Here we perform more detailed study of Gribov's copies in the classical
mechanics and determine the quantum spectrum of masses for the arbitrary spin
case.
