The ornamental design for a first stagepressureregulator, as shown and described.
FIG. 1 is a perspective view of a first stage pressure regulator showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof; and,
FIG. 6 is a top plan view thereof.
The broken lines shown in the figures are included for the purpose of illustrating portions of the first stage pressure regulator and form no part of the claimed design.