  Several quantum proper time derivatives are obtained from the Beck one in the
usual framework of relativistic quantum mechanics (spin 1/2 case). The ``scalar
Hamiltonians'' of these derivatives should be thought of as the conjugate
variables of the proper time. Then, the Hamiltonians would play the role of
mass operators, suggesting the formulation of an adequate extended indefinite
mass framework. We propose and briefly develop the framework corresponding to
the Feynman parametrization of the Dirac equation. In such a case we derive the
other parametrizations known in the literature, linking the extension of the
different proposals of quantum proper time derivatives again.
